These Security Settings are used to redirect users from one site to another site. This is most often used when you have a Portal (either the Employee Portal or Self-service Portal) where you want to give users access to submit forms, requests, or tickets, but you don't want them to get into the main site where those forms, requests, or tickets are stored.
Here is an example set up where only the Demo Sites Members group should get access to the main ITProfessional site, and everyone else gets sent to the portal:
