What is the purpose of "pip install --user ..."?

From pip install --help:

--user  Install to the Python user install directory for your platform.
        Typically ~/.local/, or %APPDATA%\Python on Windows.
        (See the Python documentation for site.USER_BASE for full details.)

The documentation for site.USER_BASE is a terrifying wormhole of interesting Unix-like subject matter that I don't understand.

What is the purpose of --user in plain English? Why would installing the package to ~/.local/ matter? Why not just put an executable somewhere in my $PATH?