Anecdotally, I've heard from a number of people that catering is one of the larger profit centers for restaurants that do it, with the dine-in service acting almost like a marketing tool, or sometimes even a loss leader.
With catering, you only have to pay for the staff you need, when you need them. You also often don't have to pay for the venue, or only have to pay for renting furniture etc.
Part of the profitability also comes from the fact that you're already paying for the kitchen, and often the venue (if it's hosted at the restaurant after hours), so you're essentially removed 30% of the cost.