My wife likes sharing her phone's location with me full-time just in case she's ever in trouble, I'll know where to look for her. I, in turn, share my location with her. She's not very specific about what kind of "trouble" she's expecting, but it makes her more comfortable knowing that I know where she is.
It also comes in handy if, for example, she goes to the store and I realized I forgot something and want to see if she's still there so she can pick it up. Or when she wants to know how soon I'll be home so she knows when to start dinner (my bike ride from work is around an hour)