I delegate out to spread my vote out (their vote value is higher) and grow my stake. The service may be voting on something I am not going to vote on myself but helps the platform grow beyond what I'm interested in. I use to delegate more than 50% because it earned me more HIVE than what I would get from my own vote, then I discover @apis.hive which has a requirement of less than 30% delegated to use their service to grow your account.