It used to be so the adhesive used to get a better key to the background, plus it's a damn site quicker if you're working with tiles under a certain size .
Because it's much easier to twist and push / pull a tile into adhesive that's already on a wall, than it is to twist and push / pull a wall into adhesive that's on a tile.
Personally i Used to find it much quicker when doing something like 200 x300 or smaller to paste up a section of wall then put tiles up rather than pasting the tiles .