Brock Lesnar is arguably a bigger star in the WWE now than he ever was before he left. He commands a ton of money to work a part time schedule. Why? Because he now has the claim of being a UFC champion. He's seen as a legitimate monster, not just from a storyline angle. Bobby Lashley, to a lesser extent, has seen the same trend in his TNA/Bellator career. He's seen as a legit fighter, and with an 11-2 record in MMA, while he's not beating top guys, he's winning the majority of his fights (and finishing them). Padding resumes is always best for business if you're in any sort of fight promotion - even the WWE can benefit from it.