Maggie's driving is the perfect metaphor for how the body creates the illusion of free will.
It feels like we're maggie, making choices that our body then does. But looking at the full picture shows that the nerve signals that initiate action happen before the conscious choice. Marge - our brain - determines what we do without any input from Maggie, and this also signals Maggie to feel that she's made a choice.
We're all just Maggie, trapped in a vehicle we have no control over, pretending otherwise.