The videocard should regulate its own speed when it gets to hot. If it doesn't, the fan is broken. There isn't really a need to manually change the fan speed.
Of course it should regulate its own speed, but sometimes that might just not be enough. And if you think it's getting to hot, why not regulate manually?
When the cards are tested at the production labs they have their own enviroment, and the cards are calibrated with respect to that enviroment. So if you place your computer in a small corner of your room the overall temperature will rise, and it may be favourable to increase the speed manually
