Though it's a platform that began with the joystick as the dominant control method, over the past twenty years the keyboard and mouse overtook it to become the only way to play most games on the PC. Which is a shame, because there are a lot of them that play better with a control pad. This has become especially true over the past 3-4 years, as the PC has started to get a lot more ports of console titles, thanks to both the relative ease of bringing code over and the increasing popularity of Steam leading to a bigger market.
While the keyboard and mouse remain the best means of controlling many titles, especially PC staples like FPS and strategy, there are plenty of other options that are really crying out for a pad.
You could go and buy a custom PC control pad, but that would be stupid. Chances are you already own a control pad you can use on the PC: the ones that came with your Xbox 360, PS3 or Nintendo console.