You poke people with your E, which does decent damage and has a slow applied to it. Furthermore, his boxes give him excellent bush control and can effectively negate the ability of many champions to engage him or his carry. For example, it is troublesome for Leona to use her combo when she can potentially E into a box next, etc. Finally, Shaco's ability to Deceive gives him a get out of jail free card, allowing him to get back to his tower from anything that does not stunlock him for his entire health bar.
He doesn't have much kill potential, but he has a respectable amount of peel if he is clever with his box placement and his free slow. And it's impossible to ignore the utility of a support that can flash, because it means you can pretty much ward whatever you want.
The problem with Shaco is that his hard CC requires a 2 second activation period. That's fine in the laning phase when you are going to be in the same area for a while, and it's decent to protect against sieges, but it's not very useful in the majority of teamfights. Other supports can peel for their carry. Shaco has to hope that the enemy Rengar will choose to let them live for more than 2 seconds after he jumps on them before he can really help. Fine in situations where the only enemy team's initiation is "hope for a lucky Blitz pull", but ineffective against a team comp with more synergy than that.