Why multiple capacitors in parallel?
A couple reasons come to mind.


Lower ESR. The effective ESR of the capacitors follows the parallel resistor rule. For example, if one capacitor's ESR is 1 Ohm, putting ten in parallel makes the effective ESR of the capacitor bank ten times smaller. This is especially helpful if you expect a high ripple current on the capacitors.
Cost saving. Let's say you need a large amount of capacitance. A single large capacitor might be more expensive than several smalls ones that add up to the same amount.
Filtering. Capacitors of different values have different impedance characteristics as a function of frequency. If you're trying to filter out a range of frequencies (noise, EMI, etc), it's helpful to put a range of different capacitors next to each other to present low impedance to as much undesirable frequencies as possible.
Stock availability. Distributors don't carry every possible value of capacitance. You may need to combine multiple caps that add up to a specific capacitance you need.
Board Layout. The physical constraints of the PCB or enclosure may allow multiple small parts to fit where a single large part will not.