I just think that instead of giving a proportion of their contracts to resellers and thereby lose a lot of money on them (cause they're going for less than the network's price, and the reseller has to make a profit too), they could use that money to lop off a bit off their own contracts and lure all the bargain hunters THAT way!
The only conceivable use for it would be if a reseller could bundle the contract with a particular model of phone which the network does not have an agreement to sell, but I've never seen that so far - certainly when I bought a contract through one of those my phone came with Orange branding, so presumably the resellers don't buy the phones separately.