Not sure which rec center you mean and when or how often you have been observing, but just look at DPR's activity listings. You may have observed breaks between sessions, but there are coop preschool programs, athletics, teen programs, programs for seniors, camps, community gardens, community meeting spaces, playgrounds, and so on. It's also a place to go to the bathroom when at the playground outside, a polling place, etc.

What kind of services should I expect from my neighborhood DPR rec center, other than their staff to stand around chatting with the loitering teens?

We have a newly renovated rec center, with lots of toddler tables and chairs inside, but nothing ever seems to go on in there for little kids, or older kids for that matter, other than suspicious loitering at the door.

What does your rec center do for you?