Moscaps are used a lot of times because they are “free” you don’t have to pay for extra masks. They also have higher capacitance density when compared to momcaps which also tend to be free. The tricky thing about moscaps in an analog circuit is that capacitance varies significantly depending on the voltage across the device. But this is okay for decoupling purposes.
You have IR drop and parasitic inductances from whatever your supply is to the ASIC, so having decoupling helps stabilize that during large transients. Even in analog circuitry, you will have supply noise because of switching regulators and other switching circuitry. Having on chip decoupling is nice because it reduces the parasitic inductances of PCB traces and bump/wires which in turn improves the effectiveness of the decoupling. Even a few tens of pF will help with decoupling. Especially at the block level. Certain blocks need it more than others. For example, decoupling on IO buffers will shunt high frequency noise to ground, this prevents it from going somewhere undesireable like a bandgap block. Decoupling at the bandgap will also shunt undesirable noise to ground too. You also need to make sure you don’t over do it too because at a certain point, it can start to impact your overall yield.