Have you ever wondered what packages you've installed via Homebrew? You know, the ones that you've actually purposefully installed: 

brew install <whatever>

I have always wanted this, for a number of reasons, but one of the main reasons is to quickly see what software I have installed on my machine.

To be clear, brew leaves doesn't actually show what you have manually installed, but it shows the commands that have no dependents. leaves is a reasonable approximation of this, though.

Additionally, I like to use the command as a quick way to show things that I can probably remove from my system without worry. Something I use that isn't in homebrew might still use the package, but you'll have that.

—

Sometimes, you just miss things in documentation. I've looked at the brew man page a number of times while this feature has been in homebrew, and I haven't ever noticed it. I'm not sure if it was a product of poor searching, or me reading it and not comprehending it. The man page says this for the leaves subcommand:

Show installed formulae that are not dependencies of another installed formula.

You live and you learn.