For example, my company (https://circleci.com) makes Continuous-integration-as-a-service, marketed directly to developers. We have a lot of companies using us with incredible engineers (Stripe and Zencoder are 2 obvious examples who have agreed to be listed on our homepage, we have many more). Why - when they can build it themselves? Because we have built a compelling product that is much better than they could do themselves (and in many cases they tried)!
To give you one example, one customer has a test suite that takes 60 minutes on his laptop. He just pushes to Circle when he wants to test, and he gets a result in 13 minutes (blazing fast build servers, plus automatic parallelization)! That would overload a single build server, so they'd have to get a cluster set up. That sounds like fun , but only for the first hour.