Win-win for UFC: draws WWE fan base (huge) and shows MMA to be the real deal. Plus, it would have been a waste of money to put him against a can. I don't think they wanted to risk a GG-CC redeux (put him in with a relatively unknown entity). If he loses to somebody good, it's acceptable. If he beats someone good, they could spin it as seeing huge potential in Lesnar because of his collegiate wrestling experience, not WWE scripting, and he's now a UFC-contracted MMA fighter.
There's no question that bringing Lesnar in was a business decision based upon the creation of new MMA fans, who were familiar with him in WWE.