Honesty.
It buys you a license to use the product you’ve already been using for 2 full versions, so if you bought now, you’d have Reaper 5 with all the updates, and Reaper 6 with all the updates, and then you’d need to buy again for version 7.
If you’re wondering how long that is, I bought version 4 in 2012, and have still only paid for it once. The Evaluation version is not limited or crippled. They’re counting on people respecting their business model and appreciating their product, and wanting development to continue. I didn’t *have* to give them $60 to use it – I wanted to. It’s worth so much more than that, especially when you consider how much Pro-Tools and other pro-level DAWs cost.