As per the definition given in the textbooks, it is the applied gate voltage such that there is no band bending in the band diagram.

Now if we really need to understand why there is a requirement of such voltage we need to understand what is band bending.

Now what is band bending?

If we go according to the standard textbooks it is the bending of energy bands in a semiconductor due to the transfer of charges.

Now again there can be many reasons for the transfer of charges in a semiconductor but in the context of MOSCAP, it is mainly due to the difference in work functions of metal and semiconductor

In the above diagram we can see that due to the lower work function of metal (eɸm) the electrons flow from metal to semiconductor that is having higher work function.

Due to this movement of charge an electric field is created between the semiconductor and the metal.

This Electric field causes the bands to bend.

But again the question arises, why not just leave them bent. Why fix it?

The answer is simple. To prevent unpredictable behaviour. If the bands are left as they are it will lead to shift in threshold voltage, can alter device parameters and affect the switching behaviour of the device.

We also need to compensate for the built in potential, generated by the difference in work functions between metal and semiconductor.

Therefore, we apply flatband voltage to ensure stable device operation and predictability.