I have heard a couple of different reasons for this. One is that their retinas are slightly off center so crossing their eyes allows them to see better. Another has to do with the way their brains interpret the images. That I didn’t really understand so I will leave it at that.
I also read that some breeders have bred this cross-eyed trait out without actually fixing the misalignment problem so if you see a straight-eyed Siamese cat, he or she may not have ideal vision.