In the series Edit
Mirabel first bought her wagon in Snowcreek as a place to sleep instead of the local Sleeping Hippo Inn. When she stayed the night in the Cyprin family household after the party defeated the Very Hungry Centipede, the party weren't able to use the wagon the until they reunited with her the following day.
From then on, she continued to use it as a place to sleep instead of the inns the party usually stay in. When she was transported to the infinite forest to receive the Effigy of Haunting from her patron, Queen Titania, her wagon was transported with her.