I hear this complaint often, especially in regards to gun control. Yes, there are a subset of people who do what they are going to do irrespective of laws. There is also a middle ground of people where the laws might curtail unwanted behavior. But the main purpose is it provides a basis for punishing unwanted behavior.

To make it concrete, one could argue that bank robbers rob banks even though it is illegal, so why have a law against it since law abiding people aren't going to rob banks. Does anyone really think we should remove such laws?