List installed packages not required by any other installed package.
leaves lists all the packages installed on your system which are not required as a dependency of another installed package. However two or more packages might depend on eachother in a dependency cycle. Packages in such cycles, which are not required by any other package, are also listed.
The list gives you a nice overview of what is installed on your system without flooding you with anything required by the packages already shown. The following list of arguments basically says the same thing in different ways: