Question

why should the resistance in two gaps be interchanged

Answer 1

The interchanging of the resistance are done, in order to get less errors or no error in the result.

The resistance in two gaps can be classified in two forms that are known and unknown resistance.

The basic property for the symmetrical meter bridge,  if unknown and known resistance interchanged, the resultant resistance will be same in both the cases.

But when the system is not symmetrical, then during the unknown resistance there will be error in the total resistance, but during the known there will be now error.

So, in order to not to get the error, there we place the interchanging of the resistance, not to get errors or less errors.