People today think they can find everything on Internet. But this is not true: the Web is still smaller than the real life. It is especially annoying when you arrive in a new place. You search for your surrounding on GoogleMaps and you find a grocery near your hotel but there are no opening hours, so you get there and kiss the closed door. Or you never find a beautiful small park with a fountain that was at 200m from your hotel because it was not on the maps and you always took another road.
So I think the fuller GoogleMaps are packed with information, the richer is our experience in new places. And even in our own familiar places. I ignored the existence of a new contemporary art museum in my own neighborhood for a year until someone added it!
Most of all, I like adding green areas and hiking itineraries. This is tricky too: you may know the location of a forest trail but you can’t draw it on the maps. So you can mark one or more entrance points and give more instructions in your comment.