I got out of "auto" on the camera dial sometime ago and moved onto the "A" or "S" (never really looked at "P" but I think I should). Played with "M" a little bit - very little. I have to use "M" for my lensbaby as it is a fully manual lens so that was by necessity.
Speaking of using flash, A,S,P modes offer metering ambient exposure, and P is often a good choice with fill flash in bright sunshine - P prevents our setting our indoor f/4 in A mode, and then wondering why we only get ERROR in bright sun (gonna need about f/11 due to maximum shutter sync speed).
But ambient level is often totally insignificant indoors, to be ignored. If we do pick some of it up, it is often orange incandescent. Manual lets us set faster than 1/60 to keep it out.
One thing that puzzels me though is how having TTL with off camera flash works. I can see on camera with one flash as the camera meters the light needs 'through the lens' aka TTL and sets the lighting accordingly to get the 'perfect' average lighting. But if I mix TTL flash and M flash and some of my off camera flashes are modified (in a softbox) or bounced how does the camera factor that into the TTL equation? And how does the camera TTl system know how close my off camera flashes are to my subject. Proximity to the subject makes all the difference.
Speaking of the Nikon Commander, when we push the shutter button (or FV Lock button) then the Commander swings into action. It individually requests and then meters a preflash from each TTL group, and sets the metered power level in each TTL group. It also sends the Commander menu Manual power to any Manual groups. System does not know how many flashes, but it individually meters each TTL flash group. A group could be one or more flashes metered as one.
The remote flashes are still TTL metering, same as one hot shoe flash. Metered one group at a time with Commander. Bounce for example, goes up and reflects and come back down, and the meter meters the center spot of the frame. Or a remote in a softbox or an umbrella, the meter still meters the light actually reaching the center spot of the frame, where it matters (doesn't matter where it has been first). If manual flashes, they are not metered, but they receive instructions to set the power level from the Commander menu.
This all happens in a split second, but all of that flashing can cause pictures of the subject blinking, but FV Lock is an easy way to work around that. Or the inexpensive Nikon SG-3IR shield will help too.
I see some systems offer 'pass through' TTL foot on the trigger (e.g. Cactus and some of the Pocket Wizards) but the remote flashes must be manual. In that situation how does the camera's TTL take into account the effect of the remote flashes. The camera's TTL system might know about the other flashes if it communicates with the trigger or if you use the on camera flash as a commander, but again, if modified in a softbox or otherwise, and proximity to the subject. The camera cannot factor that into it's TTL equation. So what is the point of TTL on the camera in a set up with off camera flashes in manual mode?
The manual triggers are easy, but I'm largely ignorant of all the TTL remote systems except Nikon Commander. Each is an individual system, with its own methods, and no compatibility among them.
But (excluding the Commander system), what the camera TTL system sees to work with is one hot shoe, with hot shoe pins to communicate with One TTL flash. Just One. Nikons multiple TTL system is the Commander.
I'm not the one to discuss the others, never had one of them. Just guessing, but like in the Yongnuo system, the remotes are in TTL mode, which is the mode in all TTL flashes that the flash can accept power level from the flash foot. Not the same action as TTL mode however, because the camera has to meter TTL, and it knows how to meter one hot shoe flash. I've been curious about them, but not enough to buy one. I'd be very interested in hearing details of how they work.
The modes I normally use are one hot shoe flash for bounce, usually TTL. Or sometimes the Commander or manual optical slaves for two remotes in quick umbrella setup. Or alternately, multiple manual flashes in a studio setting.
And one more thing, ... see very mixed opinions on whether a light meter is needed. Obviously not necessary, but is it worth the $280 I see for one that does flash? I have the app on my cell phone that gives ambient light but no good for flash.
My opinion is the camera has a very good meter for non-flash use, and that's what I use for non-flash. It is a reflected meter, but we learn to to use it. An incident meter reads more accurately, but awkwardly has to meter from the subjects location.
For hot shoe flash, again, the camera meter is very good (and still a reflected meter). I use bounce indoors, and direct TTL with compensation for fill flash in sunlight.
For one manual hot shoe flash (bounce or direct), trial and error is not hard to find the right exposure (TTL is normally a closer starting point though).
For multiple flash though (including only two flashes), the lighting ratio is the thing. To know what we're doing, we have to meter them, one way or another.
For studio flash with multiple manual flash, then a hand held incident meter seems very necessary to me. I can't imagine living life without it.
(I have a couple, but I only use them for that purpose). IMO, multiple manual flash is about the only reason to have a handheld meter today, but for multiple flash, it is a day and night difference, either guessing at the level of four flashes (at every setup), or simply taking a minute to meter them and setting each of them as we want them to be, and then KNOWING what they will do. Easy and fast and correct repeatability from last time.
I've never had any problem with a simple optical slave (speaking of indoor sessions), but I've never put one inside a softbox either.
A Sekonic L-308 is not more than $200, and is very adequate for studio flash. L-308S was previous, and L-308S-U is the new replacement (I think longer warranty may be only difference?)
See
Why would I need a handheld light meter? for more, including a description of using it at page bottom.