They are far and away the most interesting faction in the game - in terms of what they know, how they operate, and what they can offer. It would also be a faction that operates very differently than the other factions in how clandestine they are. The closest they got to "a secret group" was Fallout 4's Railroad - which was "Follow the railroad signs to the railroad museum and that's where the railroad is". The Enclave also has been exploring taming and domesticating the wildlife of the world and dealing with the Super Mutant problem.
You're literally asking, in essence, "Why would anyone want to see what's inside Area 51?" when the answer is "I don't know, but there could be cool alien shit in there".