what is a decoupling capacitor and why do we need that any way

Lot of times people talk about decoupling capacitors in the design and why do we use them.
I will jot down what i understood about them. any issues readers can respond.

We all know that for capacitor Xc = 1/2(pi)fc

For DC signals as frequency component is zero capacitor will provide very high impedance,hence when we connect the capacitor across Vcc/Gnd essentially It will not allow DC voltage to pass through it.

then why do we need decoupling capacitors any ways and how do we select the value of it without just taking standard value of 0.1micro farad or 0.01 micro farad.

This is what my understanding about it.

Take an example of any Integrated circuit:

Any IC will have lot of input/output pins and Vcc and Gnd pin.
Now why dowe connect decoupling capacitor between Vcc and Gnd.

It is because at any point of time the outputs will be switching i mean some will switch for low to high or some will from high to low.

from basics we know that Q=CV
and also Q=I*t

Hence I*t=C*V

hence C= I*t/V

you will calculate the value of C from the above equation.

where V here will be deltaV it means how much ripple voltage you want to allow for the Vcc pin.
for example if you have any IC for 3.3V, Vcc you will usually chose 0.05V or 0.1v

accordingly you will know delta V.

now t can be found from the data sheet with rise time or fall time of switching output.

I think you got the idea of how to decide on the decoupling capacitor value.
any discrepancies or any further answers to this topic are appriciated.