They do it because people don't usually like to start a new table up.
Heads up play, you are going to pay the blinds a lot more and its tough if you don't have enough experience.
So the more tables that are started, the more players will be sitting and playing. Which in turn means they will get more rake.
Generally online propping, they don't actually get paid as such. They get to play and get all their rake back. Of course this does vary from place to place and only the sites are going to really know how much propping goes on.