Degrees are usually a signal about how well the person is expected to perform. A degree from a top school with a decent GPA means that not only was this person select among many to attend, but they also managed to go through it okay. That is why employers ask for a degree in anything for certain jobs. They may not care about art history, but a degree is a good signal that "hey this person is smart."

For CS it's a little more practical but a degree is still fundamentally a signal. As is an active github account, blog, etc...