Some of it is to save roleplay time. You just assume that the cavalcade of "free" but weird stuff (including, oddly, the live spider you need to swallow for spider climb) are there with every pouch but if you lose your pouch, I guess you could roleplay looking for a spider to swallow, etc. And it is easier to have the pouch, or the feat, than roleplaying hunting up every free thing of every spell you cast, or can cast, that has a free but weird material component.

Mind you, an illusionist sans spell component pouch or eschew materials feat can get quite far with just a fleecy coat that they can take little bits of fleece off of at need.