Why are null measuring methods considered to be very precise?
There are two key benefits of null-balance measuring methods.

The first is that the quantity being measured is not affected in any way by the measurement when the system is in balance; i.e., there are no "loading" effects.

The second is that since the measurement is always made at the same point on the indicating device, any nonlinearities in the indicating device itself do not affect the measurement at all. The only requirement is that the indicating device must have good "repeatability", which means that it always comes back to the same point for the same quantity being measured, regardless of its previous history.