Roundabout subscribers can exchange a subscriber ticket for a general admission pass that they can try to use at any performance. At least, that's how it worked when I was a subscriber. GA patrons were almost always seated very close to curtain, within a minute or two of a performance's start time. As for MTC, they might have a similar option for subscribers, or as the other poster said, they could be seating volunteer ushers in those seats. Or they might be last-minute ticket buyers from the cancellation line, if a show is sold out.
"You travel alone because other people are only there to remind you how much that hook hurts that we all bit down on. Wait for that one day we can bite free and get back out there in space where we belong, sail back over water, over skies, into space, the hook finally out of our mouths and we wander back out there in space spawning to other planets never to return hurrah to earth and we'll look back and can't even see these lives here anymore. Only the taste of blood to remind us we ever existed. The earth is small. We're gone. We're dead. We're safe."
-John Guare, Landscape of the Body