Its all about control for companies like Shimano etc. Bit like retail price control in all but words. The reward for Shimano would be that Madison have to support their full range, and Madison in return do not have any competitors in the UK so can work within Shimano suggested trade and retail prices giving them healthy margins.
Wiggle & CR I assume bypass this and deal direct where they can, plus they build bikes opening up a bit of abuse of OEM branded parts.
Then you get in to bike builders getting big discounts, but not always fitting the items to bikes and sell them without retail packaging.
There are ways around it as a retailer. Schwalbe for instance controlled the prices in the UK on the One Tubeless tyre on the basis they didn't really want them for sale online as they feared people fitting them to inappropriate wheels so the prices were controlled and much, much higher than mainland Europe, a switched on retailer could buy form a large German online retailer and boost their margins. Or better still buy from a German distributor