Espionage has the same issue as Federations in that you need to invest a tradition tree to bother with it at all. Like I don't think you even get spy/encryption tech without starting the tradition.
You need 'more physics points from Researchers' techs (Quantum something?) and Colonial Bureaucracy to get first espionage tech, after that its 'previous spy tech + physics researchers level tech'.
From what I have played espionage, both with and without the tree, there's really 4 actions you would use:
Gather Intel: has a chance to give you colony sites info, lift fog of war (very nice if you didn't survey/visit their systems with Science ships) or show enemy fleets, neat before invading that pesky neighbour who doesn't wanna be friends.
Acquire Asset: It's a bit random, but Assets can be reused, and +4 to operation speed is always nice.
Smear Campaign: Has potential to break those pesky defense pacts your neighbour has, making it easier to wage war.
Steal Technology: At worst it boosts something you're already researching; at best it will give you a tech that has been eluding you for those XX years already.
Subterfuge Tradition tree is nice (+50% infiltration speed, intel refunds after operations), but not a requirement; you easily get more codebreaking/encryption bonuses from edicts, techs and civics, really.