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well if you're going to shoot at f5.6 all the time, then you could say no point at all really. The amount of light let in is a factor of the focal length of the lens , the aperture and shutter speed.
The aperture is a ratio based on the focal length of the lens, it can get quite complicated explaining it and I'm not technically-minded enough to bother much with these things, I'm sure someone else can explain it if you're interested.
You should be bothered about apertures when considering the aesthetic quality of your shot in relation to depth of field, 2.8 will show less depth of field than 5.6 but will allow you to shoot at a faster shutter speed, so it depends on whether shutter speed or aperture is more important in the circucmstances of the individual shot you are about to take. And this dictates whether 5.6 or any other aperture is more important to you than another aperture.
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