In the past you would have to give your friend full access to everything on your phone if they asked to use yours. A simple call or a simple email could turn into a device scouring session. This is why you need the multi users option. The multiuser option allows you to log out of your account and allow your friend or family member to log into their own separate account with no access to your personal files. Just flash the file in recovery and you are good to go. If you don't like it just flash the stock file to restore the stock setup. Head to the link below for the downloads.
Via XDA