@The-Darkin-Blade Good question! It's just for organizational purposes, actually. We want to sort the different ways that we are trying by setting $$x$$ to be the shortest side.

If we didn't have this requirement of $$x \leq y,$$ then we might have $$3, 5,$$ and $$6,$$ as side lengths on our list of things to check in addition to side lengths $$5, 3$$ and $$6,$$ which are really the same combination.

So, in a nutshell, this requirement ensures that we cover all the cases without doing any of the cases twice!