why, when doing probability and rolling dice with inclusive numbers.... do you subtract 1 ?

Answer 1

generally you don't subtract 1, you subtract FROM 1
The prob. of anything is a number between 0 and 1, so often when there are many cases to consider, it might be easier to calculate the prob of the exceptions, then subtract that from 1.

e.g. What is the probability of throwing a sum other than 7 when tossing two dice.

We could find the prob of throwing a sum of 2, a sum of 3, ... and then add them up, or
we could find the prob of a sum of 7, which is 6/36 or 1/6, and subtracting 1/6 from 1

prob of NOT getting a sum of 7 is
1 - 1/6 = 5/6