Therese often makes cardboard sets for her portfolio as she looks for a set designer job in ballet or Broadway. A set is an imitation of real life, and Therese's models are imitations of imitations—whoa—which is kind of like how distant from real life Therese is.
Carol insults Therese's designs, saying, "That's amateurish, isn't it?" (13.102). She thinks Therese needs to actually see the world before she can replicate it, which is one reason she takes Therese on the road trip.
On the trip, Therese falls for Carol and says, "I wonder if I'll ever want to create anything again" (16.110). This offers a key insight into what drives Therese to create the mock sets in the fist place. It seems her creative drive comes from a place of wanting to create a world for herself—and she finds this world with Carol. Therese doesn't need her little models anymore when she has the real thing.
The real thing doesn't last, though, and Therese continues her stage metaphors when Carol returns to New York:
She felt like an actor […] as if she had been playing in these last days the part of someone else, someone fabulously and excessively lucky. (16.125)
Therese feels like an actor in her life with Carol because her new identity is only temporary—Carol leaves her in hopes of maintaining custody over her daughter. This, too, is temporary, though: Carol loses the custody battle and winds up working and living alone. She invites Therese to join her, and while Therese initially declines the offer, it doesn't take long for her to return to Carol. Perhaps their first go round was simply a dress rehearsal, and after a bit of practice, Therese is now ready for the real deal.