Spot Metering will insure what you are focusing on will be properly exposed.
There is so much more to Spot metering.
Spot never works like we superficially imagine. Spot is NOT a beginners tool. Spot requires we know more.
Spot does Not insure the focus point is properly exposed. Spot only insures focus point
will be exposed to be middle gray tone, which is what meters do, which might be proper, depending on the spot selected. Maybe Spot was on a gray card, that's good. But more likely will not be proper exposure, depending on the spot. The normal idea of Spot is that we know how to compensate for the spot selected (standard procedure is, if spot metering on a face, faces are not middle gray, so open exposure one stop more, etc).
Except, Spot only affects
ambient light exposure, does not affect flash exposure. Spot does switch flash from TTL BL balanced mode to be TTL mode, but flash uses its own metering area, ignoring Spot.
In dim ambient, where we need flash, the ambient is way down and insignificant, so there is no issue with Spot (and ambient). There is little advantage of Spot either, except we do get TTL mode, which can be a bit brighter, and which can be a good goal, esp in normal indoors shots at home,
assuming low ISO.
Except high ISO possibly brings the ambient exposure way up, so now there may be something bright to balance with flash.
But Spot and TTL won't do it, no balancing occurs in TTL mode. TTL flash will overexpose the subject instead of backing off to balance with it (the sum of proper ambient and proper flash exposure is 2x exposure). So manual -EV flash compensation will likely be required then. Also probably need CTO or other filters on the flash to match the ambient white balance we see now.
High ISO and flash really don't mix well (and Auto ISO and Manual flash is especially bad). Instead we solve the dim problem with flash power to make it not be dim. If flash does not have the power capability (bounce), THEN a little higher ISO (but still not maximum ISO).
Spot metering requires more knowledge than point & shoot.
Details about Metering Principles