i think it is the same reason with the girl did it before..The girl had gone that house for a little peace(sleeping on the sofa) and wanted to memorize the things they've shared..But i think the major reason is to test people with the ability he had gained in prison..He entered to that houses in an invisible way, that nobody saw him..He tried all the houses,and tested his ability and stamina that soon will lead him the "main man"..Girl's husband..That's because he tried all the houses and people live inside, and than wanted to face that man..it would be revenge for him..(and heyy he's mentally-retarded that's the point!!)...but'it's really a good movie that took away my pain..nicely done.
I think, if anything should have been left out, it should have been these visits (except the final one of course). The shakey camera angle as character viewpoint was so cliched.