Official Luthiers Forum!

Essentially you will have variability in each coat you spray (orange peel). If you level after every 3 or so, then you reduce the variability before the next set of coats, etc. etc. However, if you don't wet sand every couple coats and just sprayed 10 coats and then tried to wet-sand, the variability would have compounded the unevenness and making your surface irregularities more volatile. This therefore makes the finish harder to deal with, level, and making it perhaps even more risky for sand-thrus because of the variability despite the added thickness you start with. Kinda like dropping two pebbles into a pond and in some places the waves match up to double the height, in some they match up to double-reduce the height. KINDA like that... but less uniform than waves (more dependent on random placement of orange peel). It's not a perfect metaphor, but it helps demonstrate.
Chris