What is the purpose of getting good grades once in college?
The other answers give the standard practical reasons why one should aim to get good grades, but I prefer a more idealistic explanation, which is that good grades are not the real goal one should focus on, but rather a side effect. If you focus on the real goal of learning the material at the highest level possible, good grades will follow automatically. So, good grades do have their practical uses, but philosophically speaking, they are just a distraction.