It is a western pleasure thing. Basically, the ideal for a wp horse is to go on as loose a rein as possible...which makes a clean stop more challenging, especially on a young or less trained horse. Thus, the spur stop, which is a more noticeable aid. At least that is my understanding of it...not sure what else you guys use it for:P
So, you are, in effect, banned from using rein contact, so you replace it with something that is also an aid to go? Sounds odd and confusing to me. I'm sure it must be more complicated than that, right? I thought western placed a lot of emphasis on the seat and legs to give aids.