Its not just any old dumb rule. Say I wanted to make something really unlikely to be divisible. Well, let's try taking something very large and very evenly divisible, and then add or subtract a 1 so that it's always just a tiny fraction off from being divisible. In the addition case we might get something like 10000001. In the subtraction case for the same number we would get 9999999, after which we could factor out 9 to get 1111111. You could also do it with numbers that don't line up with base 10. 360 is a highly divisible number (hence its use for angles) but 361 / 359 are going to be 1/360th out of step with those divisors (and in fact, 359 is prime).

So, for this reason, a repeating sequence of 1s is a natural choice to try and find prime numbers.