My guess would be to prevent middlebox fiddling. Such as an end user device asking for 4k, but a middlebox somewhere between the customer and Netflix inspectin

My guess would be to prevent middlebox fiddling. Such as an end user device asking for 4k, but a middlebox somewhere between the customer and Netflix inspecting the http content and blocking it (or modifying the client request in flight), because "only 480p allowed here" or something like that.