One's influence on the social network is determined by one's Steem Power. More Steem Power means more influence--meaning your up or down vote counts more than someone with less Steem Power. Also, the amount of one's Steem Power factors into how much curation award one receives.
So, since people want to be socially influential and they also want meaningful curation awards, they want Steem and are willing to pay for it. Hence value.