To my understanding, bandwidth is something of a rationing mechanism to prevent prolific spamming. Every time you commit something to the blockchain, it uses up a bit of your bandwidth allocation. Bandwidth recharges, so you can come back later and comment or post or vote if you happen to run out. Having delegation of bandwidth could also presumably discourage abuse, since the delegator could remove the "juice" if some app or some person went off and did bad things. For whatever your definition of bad is.