Why do we have to change the sign in case of subtraction? Is this necessary ?

Take an example of two integers 4 and (– 3) and suppose we have to subtract (– 3) from the 4, i.e.

If the don't change the sign, here then the above process results as:

4 – 3 = 1, so the current result is quite different from the result we obtained previously.

So, we have to change the sign of subtraction, according to the sign convention given in the bracket above.