I'm not that sure. In the books, the original stories, Sherlock does say that Mycroft is, essentially, the British government. He doesn't seem to have much part in taking down terrorist groups, and the only time he ever has part in a case is when he tells Sherlock there's a case. In the BBC drama, Mycroft seems to be more of a person who takes care of secret matters, like taking down terrorist groups. He also mentions he has ties with the Secret Service, so maybe something to do with that, I guess?