Signing out and Signing back in can have several functions, It can get you out and back in to a PC that is Crashed or has frozen (Ctrl Alt Delete Sign out).

It can also be used if your PC has more than 1 user account (sign out of one user account and signing back in to another)

It can also restart Explorer Without re booting, i like to think of it as a mini reset should i need it without a full re start.

You can also sign out if you are in a work setting and leaving your desk, be sure to save any ongoing work

I am sure it has more functions, others may list, good luck.