I use mine much like unimogbert does. I turn it on, mark points and turn it off and your car IS the most important point you mark, trailhead, turns and forks are good too. If you can mark a point on the GPS that is a good visual reference, you will have a point of reference from the GPS to the real world (this may be more relevant to non-mapping units like mine, but it's still of use to units with maps).

I use it to fix specific locations, rather than plotting a track. If for some reason I became not found, I'd pull out the GPS and determine bearing and distance to one of the locations I'd marked. Then I'd use a real compass to walk to that spot. This saves batteries and keeps me from becoming fixated on the GPS rather than watching the trail.