I have a stroller that I modified to stack two hard plastic carriers on for when it comes to annual check up at the vets time.
As Suzanne pointed out, some people like me don't have cars, and carrying two unhappy cats in carriers is a bit stressful, even if it's just a 10 minute walk to the surgery.
The stroller did originally come with a separate lift in/out sports bag type carrier, but Milo was nervous and peed in it. Despite the vet bed and copious newspaper underneath, the pee managed to seep into the cloth seam and no amount of soaking would get the smell out.
However with a steadying hand on top, I can stack two Atlas carriers one on top of the other and just wheel them round to the vets.
The boys would actually fit in one carrier, but living alone I've never mastered the knack of getting cat number two into the carrier without cat number one managing to get out of it...