The point of free speech is to be free. Pushing the line constantly is a tactic that some argue (or rationalize) is a way to maximize that freedom, but it's not the purpose, any more than the purpose of a right to bear arms is to get as many questionable shootings as possible.
Now that I think of it, you're right. I shouldn't have used the word "purpose". A proper reformulation of what I meant would be that free speech will, as a property of its existence, push the line against someone's expectations somewhere at some point in time and that denying free speech many decades ago would have inhibited several rights and expressions of rights we take for granted today.
