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

--user installs in site.USER_SITE.

For my case, it was /Users/.../Library/Python/2.7/bin. So I have added that to my PATH (in ~/.bash_profile file):

export PATH=$PATH:/Users/.../Library/Python/2.7/bin

pip defaults to installing Python packages to a system directory (such as /usr/local/lib/python3.4). This requires root access.

--user makes pip install packages in your home directory instead, which doesn't require any special privileges.