When a neighbor visits your Springfield and taps on your houses, the neighbor will earn $16 and 2XP per house.
The immediate effect you can observe is that you will see the interlocked hands symbol over your house and when you tap it, you'll see the name of the neighbor appear.
But what other, if any, effects are there for you? From what I can observe, the tapped houses don't pay out more money. So, is there any benefit for you?