Why do track bikes vary between events? Watching the Olympics, I've noticed that track bikes seem to vary between events. 
For example, in the Keirin, all the riders have bikes with a solid/disc rear wheel and a front wheel that has a solid inner but with gaps. In the Omnium, however, they seem to ride bikes with two complete disc wheels.

Some events also seem to call for aero bars on top of drop bars, some only drops. 

I can see that these might affect how the bike handles, and favour either top speed or maneuverability, but, as I only have a basic knowledge of what each track event involves, I am struggling to piece together exactly why each event has its particular combination of components.

Is anyone able to give a run down of how what the features of the bike are for each of the main track events and why those features are ideal for that event?
Regarding handlebar selection, aero bars are only allowed in the pursuit (individual and team) and time trial events (1km TT, etc). All the other track events are drop bars only.

Wheel selection is a little more dependent on the rider's preference, though this is regulated as well; dual discs wheels are allowed for track time trials (essentially the same events that allow aero bars), while a deep section front wheel is always allowed.