Aesthetics, mostly. But it also is a visual reminder not to over-complicate things unless I really need to. I got the idea from reading some source code from the suckless.org guys.
I meant to document the directory structure here:
(h)eaders, (m)odules, (e)xamples, (t)ests.
Arguably obfuscated, but I think with only 4 directories, it's still manageable.