Steem has value because people are willing to buy it. Here are some reasons why people buy steem:

• People simply wish to support Steem, the Steemit platform or the authors, the way to do that is to buy steem.
• Power-hungry people buying Steem Power to get a bigger sway on what's visible and what's not, who earns money etc, and more popularity. (I've bought some steem - what a wonderful thing, I can press the upvote button on my own comment and it's the TOP comment! Whoha!)
• People buying in. believing steem will grow to the moon. (This is not sustainable - if this is the biggest reason people buy steem, then it will only grow into a bubble and collapse). (Plus, of course, steem is a very bad investment objective due to it's hyperinflationary nature. I think many people don't realize that and are hence wasting their money on buying steem).
• Altcoin traders buying steem, hoping to do a profit by predicting the price going up through "technical analysis".
• Many believe in further price growth because the SteemIt platform still has a lot of growth and adoption potential.
• Some people believe in long term price growth because the Steem token and Steem blockchain has a much bigger potential than what's visible in the current SteemIt platform.