Platform A has a 40% fee and requires price elsewhere to be the same. Manufacturer X sells their own product on their own website, at +40% their price.
This is bad for buyers and competition. Platform A is already big and important enough that you can't skip it, and can drive up and control pricing generally.
If the requirement were not there, if platform A does not offer enough plus service for the 40% margin, other platforms would keep the prices at a reasonable level. People could buy from the manufacturer at their original price.
A marketplace important enough that you can't skip it being able to dictate market conditions is how it manifests itself further as the primary player and controlling instance.
To look at paintings?