A conveyor belt fails at its weakest point, and on a well-specified belt that point is the joint. Buy the roll from one vendor and the splice from another, and when the joint opens each will point at the other — while your line stands and the tonnes don't move. The tender saved four percent on the roll and lost it forty times over in a single dispute about whose failure it was.
No seam in the responsibility
When the same company supplies the belt and makes the joint, there is nothing to argue about. We splice what we sell, log every joint against the belt number in the register, and stand behind carcass, cover and joint together — one phone number when anything at all goes wrong, and no letter explaining why it's the other vendor's problem.
It changes what gets specified
A supplier who will never touch the belt again quotes the thinnest cover that meets the tender line. A supplier who must splice it — and answer for the joint at 2 a.m. — specifies the DIN-M cover and the carcass the duty actually needs, because the alternative is their own crew back on site. Accountability is the cheapest quality-control system ever invented.
Ask any belt vendor one question: who makes and warrants the joint? If the answer isn't the same company, the warranty has a seam in it — and belts always fail at the seam.