Another point to add, ANY flash can be very useful, you do not need iTTL, and in fact fully manual control is easier for consistency and full artistic control. So a used Nikon manual or semi-automatic flash is just as effective for portraiture, product, macro, or commercial. In fact shooting with camera exposure on full manual is the easiest way to be sure of what you expected and is very fast once you get used to it. I picked up an old Vivitar flash from the 70s at a garage sale for $5 and it is just as effective in manual as the $600 SB-910.
The only cautionary note, most really old flash units were designed for cameras with hardware shut switch contacts connected to the hotshoe that shorted the primary of an induction coil that caused the magnetic field generated by the coil to collapse. That collapsing field induced a very high voltage in the secondary that triggered the flash tube. It was simple and reliable with only big capacitor and step up induction coil/transformer to generate the several thousand volts needed for the flash tube to fire. Those switch contacts had several hundred volts across them. If installed in the shoe of a modern camera that old flash would have that several hundred volts across a switching transistor in a modern camera that was designed for less than 60 volts, and the trigger control voltage in modern flash and cameras is TTL level, 5 volts.
A breakthrough in control came with the addition of a Thyristor in flash unites to do the switching with a small gate control voltage. Thyristors are mult-junction silicon diodes that act as a switch with a small voltage applied to one of the layers of the diode. Flash units with that were labeled with that name since it was a selling point. Those are OK with modern cameras. You can still use a modern camera with old pre-Thyristor models with the addition of a little shoe mounted adaptor that performs the same duty as the Thyristor, isolating the camera from the higher flash voltage and responding to the 5 volt control signal. My $5 Vivitar had a label "Electronic switching" which meant low voltage control signal. The going price of the very well made Nikon SB-28 that has some auto functions, is only about $25-30. They last forever.
For casual shooting, iTTL auto exposure is good, but the mode that really is cool is iTTL BL which stands for Balanced flash. That works in Nikon Matrix metering mode that meters the whole scene with a little bias for the focus point. The BL feature relies on the fact that there are two independent metering systems in play. The scene exposure is handled by Matrix and sets camera Exposure Triad settings for proper exposure of the scene. But the flash as a mind of its own, it using the same sensor but reads the focus point spot meter reading and adjusts the flash output to properly expose that spot. Imagine taking that common image of your friend with the setting sun in the background or in a room where there is good background light but your friend is between the camera and bright background. In Matrix mode, the bright background is going to determine exposure which means your friend is very dark, under exposed. You could switch to spot metering on her face but a bright background will be badly over exposed. Instead, you turn on the flash, matrix sets camera exposure for the bright scene and your flash only meters the focus point so adds light to properly exposure the person while the camera took care of properly exposing the scene. Almost magic. Nikon cameras and Nikon style flash do that really well. But so do all the 3rd party flash units nowadays. The TTL BL mode on my Yongnuo 568ex flash does it perfectly also.
That is the first function for which you might really think it is a pretty smart system. But switching to manual exposure mode the opens all sorts of options. Say you are in a party and you want a little of the ambience seen in the background but the subject is your friend in in the foreground. Easy to control the relative brightness of the background and independent control of the subject exposure. Easy, just meter on the scene in M exposure mode, and decide how much of the room you want exposed. Say, 2 stops lower, that gives the colors, and feel but not distracting from the subject so you dial in two major ticks of the meter to the left of the center meter line. Either aperture, shutter or ISO or any combination you want for artistic reasons, stopping down aperture increases the depth of field that is in focus, so if you want it blurred, open the aperture wide while lowering ISO and increasing shutter speed. or any other combination of the Triad control of exposure to under expose the scene by 2 stops. Now, your flash will meter the focus point ignoring the matrix scene calculations and expose the subject properly by setting its flash power to expose the subject well. Take the photos and your subject is properly exposed and the background is clear but not dominate in the image. It gives a sense of the ambience without competing with the subject.
You can the same by brightening a dark scene by a looong shutter. And with focus point on the foreground subject, the Matrix metering wll bring up the exposure with time the shutter is open so the background has rich saturated colors and almost fully exposed say at 1/10 of a second. Normally you or your subject would be blurred by motion at 1/10 of a second but subject exposure will be due to flash, and it metered spot for a very short flash pulse, 1/10,000-1/20,000 of a second that freezes the subject even if moving fast, and properly exposed. A very long shutter speed for room ambience is called Dragging the Shutter. In dance clubs I often shot at 1/6 sec and ISO 400 with f/2.8-4 to get the foreground subject in crystal sharp focus and the background filled with light trails and color streaks and blurred background dance movement that displays the party ambience but the subject in razor fine focus.
TTL BL mode is very cool and can work great for balancing a rising moon background with a subject in the foreground or in any scene where the background has a different exposure requirement than the near subject. You can do the exact same thing in full manual mode for flash and background exposure. After a few hundred shots like that, you can just glance at a scene and know how much the background has to be exposed for for your intended purpose and how much flash you need to dial in of flash power.
Flash and strobes are very fundamental to creative and art photography. Anyone who avoids flash because they see bad examples is about the same as buying a Ferrari Fxx and never taking it above first gear. They missing the real value of what they have.
Flash is of course not the only light sources, just as important for outdoor shooting are gobos, scrims,gels, snoots, flags and reflectors, all possible to make yourself and get pro results. But for another post.
