Please help me understand filibusters

[RobbyPants]

So, what's the real point of a filibuster? I mean, why do we have them in the first place? What are they supposed to accomplish?

I remember learning about them in school, and I never really understood them then. Is it to give the minority party a bit of leverage to keep the majority from running the table? Is it because they didn't have a good way to define how much time to discuss a topic is "reasonable", so they didn't want to impose an arbitrary limit? I have to be missing something.