Quote Originally Posted by mpenza
Several reasons:
1. With a faster lens, you have the option to use the wider aperture in the following scenarios (just a few of the top of my head):
a. available light shooting at lower ISO to minimise noise
b. need for fast shutter speed to freeze subject
c. need for longer flash range
d. shallow depth of field for creative effects

2. Also, a F4 lens will usually be less sharp at F4 than a F2.8 stepped down to F4, so you "gain" sharpness with a faster lens.

Can I add another one?

- to capture more ambience lighting and allow more versatile fill-flash control.