When does travel time for a job become work that should be paid?

My location of work is: "(address of my employer) and the greater area)".

Every day, I meet my coworker at a pre-defined meeting spot (we arrange, just a random street). We then drive to different sites across the area.

I might meet with him at 5.20, then arrive at site at 6.

On the way home, we might leave at 4, arrive back to my car at 4.40, then I have to drive another 20 minutes home.

Employers want us to log hours from when we are at site. (6-4).

Whats the deal with this?