Which is what Git does: every commit actually is a directory/file tree and a small note a.k.a. `commit message'. You could re-create the effect by hand, by placing `.commit' file in root of the multiple directories you speak of.

However, Git (and any other DVCS) does the bookkeeping and keeps you from missing that one small detail at 4AM.

Also, it's much faster to git-commit than to copy 150MB of sources by hand; mostly because Git avoids work where sensible. Ditto for git-diff.