Basically different distribution channels (speciality shops, big box marketplaces, and ecom stores) have very different levels of overhead, so if each channel was allowed to set their own price, you'd end up with brick and mortar stores doing a lot of showrooming and then online stores gaining the bulk of sales because they're cheaper (because their overhead is low).
This pretty much happened in the early 2000s-2010s so over time brands became VERY particular about enforcing MAP.
This is what I see happen in Poland with clothes and electronics stores, but I don't exactly understand what MAP is supposed to be solving here, given that the brick&mortar and on-line stores are literally the same entity/brand, and in case of clothing, they're also the manufacturer brand?