* They provide assurance against bad decisions. Every exec wants to have someone to point at when a few millions were wasted. Don't get me wrong here. The job of a higher-up exec probably implies wasting a few millions every now and then by taking the best course in adverse circumstances.
* They provide a neutral judge for execs surrounded by warring factions - and they don't even have to be internal. At our company I see how A is a sane option, but B is used by a competitor. Even if said competitor bleeds money as a result, we regularly hire the same contractors, and they love reusing their investment and see you bleed money (to them) by giving you also B. What do you do while they whisper in the ear of your boss?
When companies get big enough to become vulnerable to sociological dysfunctionality, Gartner can help a lot. Or make things much worse.