Some perspectives would say that they serve no real purpose other than performative wealth display and distribution. They appeal to everyone at fundamental psychological levels to "fit in" with a popular trend or "in group".
Their actual quality is often no better than other manufactured goods. It is their perceived quality and style that are the entire reason their brands exist.
(and... I can admit that certain "luxury brands" are definitely appealing to me personally, even if they make little "logical sense" to own - maybe not clothing so much, but... watches...)
(And many of these large shipments do not end-up as donations by the time they get to their destination, but are actually sold by weight and then resold again)
But yes - distribution/logistics of donated goods needed to those who need them should be a "solved problem", but unfortunately it is not - regulations could help. (In countries/regions where governments actually WANT to regulate and then subsequently FOLLOW the regulations rather than cancel, ignore or throw them out entirely... Pretty sure everyone knows which country I am referring too...)