Why is moving up in weightclasses something that seems so requested and acclaimed?
I don't think that boxer should go up in weight to prove their worth or that they are the best.
Moving up in weightclass seems logical to me in many cases : when one boxer starts his career relatively young and naturally sees his body and frame develop and growing out of the weightclass he initially started at; or when a boxer is intentionally a weight bully and is cutting tons of weight to remain in that weight class where his size gives him a considerable advantage, looking at guys like Hurd, Munguia or Spence. In both cases, naturally, at one point boxers will have to move up in weight because its not sustainable for them. In the lower spectrum of the weightclasses, the difference are so minimal that it also sometimes makes sense to move up in weight, either to feel more comfortable or to develop different aspect of your game: gain in power, explosiveness.
Seeing multi-weight world champions is almost common those days. It is impressive as it shows the ability to become a world champion even in a weightclass where you would be considered as a small guy. At the same time it is not that impressive to me as for most of those champions cruise across weightclasses winning world titles without ever actually challenging the best of the weightclass...