By that reasoning, all ways of grouping, analyzing, and considering numbers are discovered. And those ways are infinite. That primes are important is of our choosing.
There are a number of practical social situations where an understanding of divisibility can be useful.
For instance, say a group of social animals of equal rank stumble upon some berries and split them amongst themselves. It would be beneficial for them to understand that if the number of berries is relatively prime to the number of them, then an even distribution is impossible and they should accept that.
I'm sure more compelling examples of grouping in social animals arise all the time. Now if these animals begin to study numbers abstractly, one of the first things that they will practically need to understand is the rules that govern groupings. This will lead them immediately into the idea of prime numbers (and by extension relatively prime numbers).
I can't really think of a technique of grouping or looking at numbers that wouldn't at least implicitly require prime numbers.
It may be that higher level analysis of primes (like in this article) is somewhat artificial, but I'd maintain that the idea that a number is prime, or at least the concept that two numbers are relatively prime, is one that is fundemental to having any understanding of a number system.
