Low F- Stop doesn't totally means a faster lens as in the lens is faster in what ever way.
Lower f-stop just means more light can enter and so more brighter
It means you MIGHT be able to shoot in low light without flash
It means you MIGHT be able to shoot with a faster shutter speed
It means you MIGHT be able to shoot with lower ISO value
The above mention are just some example. Most important is that you have to be used to your gears and light. Understand how light works
Now all theory but no example is a bit hard.
Lets say you are shooting a night scenary. You might use a lower f-stop value rather then the higher ISO value to shoot so that it is not that dark
But if you are shooting a night scenary with a model (might be your gf) infront, you want a nice bokeh. You might think that using just lower f-stop value u can shoot a nice picture. You are wrong, cause light travels from the background to your model back to your camera. what you might see is a nice background and a heavily underexpose model. So you need a flash, preferable a flash with a bounce card. Don't worry about the background as I nv see a flash that powerful yet.
Hope it helps
F- stop fomule and theory can be found at
http://en.wikipedia.org/wiki/F-number