Well it’s really that agencies and studios are the only permanent institutions in that world with a big enough headcount to offer any internal advancement possibilities (and they’ll be looking for skills and background you would develop at an agency or studio), and that was the entry level. Delivering mail at least started you learning who everyone was and what their place in workflow is. That was really back before the 90s rise in internships, but the big agencies were still developing talent from that pool when I was in LA