Is the "Cellar" folder only used for Homebrew? Is the "Cellar" folder only used for Homebrew? If I am not using Homebrew, I can safely remove it?
The script for uninstalling Homebrew removes it completely, so it's probably safe to remove it.
https://github.com/mxcl/homebrew/wiki/FAQ

How do I uninstall Homebrew?
If you installed to /usr/local then you can use the script in this gist to uninstall — it will only remove Homebrew and the stuff Homebrew installed leaving anything else in /usr/local alone.
Provided you haven’t put anything else in Homebrew’s prefix (brew --prefix), you can generally just rm -rf that directory. This is because Homebrew won’t touch files outside its prefix.