For example they decide which hotels/restaurants to show in an area when you search. They won't show all of the nearby ones. They might show one that is 5km away and not one that is 100m away.
Once I was in a restaurant that was not in maps. I decided to add it to the map when I was home.
Turned out the place was in their database, but they just decided to NOT show it, even at maximum zoom level over it. But when I added it, they asked if it wasn't that other one that already existed.
Moral of the story: use openstreetmaps.