I think Robert's explanation above is confusing and misses the mark.

First, stack and reach do not describe fit, but frame sizing.

Fit is about your position on the bike, and where things (e.g. bars, seat) need to be for you to be comfortable.

Stack and reach tell you how a frame is sized, which gives those knowledgeable enough to know where the seat and bars need to be, an idea of how that's going to happen. Specifically, which stem rise and reach, and seat post extension. Even seat post layback and seat position relative to BB are described by measurements other than stack and reach.

The benefit to knowing stack and reach is not to do with fit, but rather to better understand how frames compare in size, as traditional sizing designations (i.e. seat tube length) never told us much, but is even less relevant today than ever (due to things like compact geometry and new tech that make shaping and building frames more fluid).

So rather than thinking Size 54c (which tells you zippo), think stack and reach. If all frame builders used stack and reach, getting a bike closer to your ideal size would be much easier.