She is a member of a nonhuman species that evolved on a planet with low gravity. They evolved in a way that makes then physiologically incompatible with the gravity that most other species experience. It's an example of how a person's level of physical ability and required accommodation often depend on the environment. It also challenges the viewer to reconsider whether she should be treated like a disabled person.

You're thinking of Ensign Melora Pazlar; her home planet's gravity was less strong than what most humanoids are used to so she needed the chair. She was outfitted in a regular wheelchair because the Cardassians (as you might imagine) don't care much about disability so the station had no way to be outfitted for use with an anti-grav. chair.