"The man walks to the front of the car."
Translation:남자가 자동차 앞으로 걸어요.
I suppose it is because the direction where you go is implied. Otherwise it would just mean that the man walks in front of the car and not to the front.
If you use 는 instead of 가, it would be emphasize subject - 그 or contrast with others. 남자는 자동차 앞으로 걷고, 여자는 자동차 뒤로 걸어요.