Could you give some examples as to the benefits of building a killer PC?

What interesting things can you do with it, that you previously couldn't? Or is it simply the speed and the ease of use?

I'm also thinking of building a desktop PC, but I'm afraid that I won't be needing it. I don't game, I mostly write programs and browse the web. Does it really change things in terms of latency and if it does, does that have an impact on the relationship with your computer?