Bypass caps on RF board: why are there three different size caps in parallel?
Given a dieletric type, the smaller the capacitor, typically less parasitic inductance it will have (better response at higher frequencies), but also less capacitance. You can mix sizes, values and types of capacitors to achieve a required response that is broader than what a single one can provide. It's not just about the capacitance value.

These images sum it up pretty well:



From "EEVblog #859 - Bypass Capacitor Tutorial".

And



From "Intersil - Choosing and Using Bypass Capacitors - AN1325"



From "TI - High-Speed Layout Guidelines"