If the camera has only 1 central AF point, it is difficult to focus on an off-center subject ( without recomposing after autofocus ) so having 3 AF points seems really useful. Similarly I understand the advantage of having maybe 5 or event 9 AF points.
Nowdays flagship DSLRs have huge number of AF points. For instance Nikon D800 has 51, Canon 5D Mark III has 61. What is the advantage of this? Using 51 AF points really increases the chance that the camera finds out my intention and focuses on my real subject instead of something else next to it?