The reason certain sites and people buy play money can all be summed up in 2 words, Bot Testing.
You can not test a poker bot at low limit play money tables. The play is simply too erratic for any good data to come from it. As has been pointed out multiple times in this thread, the play is more realistic in high limit play money games than it is in real money low limit games.
You can not test a poker bot on real money tables. Until you get the tweaks right, your bot will lose you a lot of money as well as be very easy to spot by the sites software. Very high limit play money tables are not scrutinized nearly as much as the lowest limit real money tables.
This leaves only high limit play money tables for programmers to get quality data on how to tweak their bots to make the most money without being able to be detected. For them to do that they need millions and millions of play dollars to test with. They simply do not have the time to sit there and try to earn that much play money for their bots to use, so they do the next best thing. They buy it.