Might be a few things.
Perhaps she is uncomfortable riding in a car with a stranger. While you might have talked before, that doesn't necessarily make you a friend or as proven trustworthy in her eyes. Remember that different people require different things before they trust people.
She might enjoy the walk.
She might do something on the way on her walk, such as visit a shop or go somewhere.
She may also visit a friend or member of family on the way.
She may also be in a regular routine which brings comfort.
I don't know when she arrives at school, or when your school starts or bus arrives at the bus stop or school, but she may arrive early by walking and therefore get in time to chat to friends or complete homework or other routines. (My brother does this and leaves early all the time to walk to school/college, despite being able to leave later to be driven in by my dad)
It may also be a method of exercise for her. Walking is healthy.