instructors don't want soldiers scattering blank ammunition all over training areas
Brecon pouch is great for scattering empty mags all over the training area when people haven't had their belts done up tight enough, spent more than a few hours digging through heather looking for them (never mine I might add)
Everything said so far has been spot on in my experience, trying to get a mag back in a pouch while stressed and/or prone is not easy, even worse when cold and wet.