Kender have pouches for the same reason that chipmunks have cheek space, it's just natural. Pouches are a vital part of every kender's life. It's how they survive. There always seems to be some item, (usually not theirs), hiding way down at the bottom of every pouch that ends up saving the day. But for every important item there is a long list of items that are totally useless, (at least according to non-kender).
Kender do not value objects in the same way as other races. Given a choice between a large chunk of green glass and a diamond, most kender would choose the glass and leave the diamond. The glass is MUCH more interesting to look at. You can never tell what will pop up next in a kender's pouch.
Kender have even created a day to celebrate their pouches called, Thanks Alot Day. Almost every item in a kender's pouch has some story behind it as to how they found it, or who it use to belong to. One thing is always certain, the items will never be boring, and they will always find more.
Some of the most valuable pouches are magical pouches. There are a wide variety of magical pouches despite their rarity. A kender with a magic pouch is to be feared and most people steer clear of them. The unpredictability of these pouches can be anything from automatically "borrowing" whatever the kender may be thinking about. Any party of adventures coming across a kender while traveling should always check for magical pouches on a kender. It would be the wise thing to do.