Understanding why people will buy STEEM is a mind bending exercise in economic and social theory. All value is perceived value, and people will buy anything they perceive will have more value in the future than it has today.
Lets start by discarding useless or oversimplified economic theories:
Myth of Intrinsic Value
There is no such thing as intrinsic value, not even with gold. The idea behind "intrinsic" is that the value is a property of an object rather than the perception of an individual. A basic understanding of marginal utility will show that 1 oz of gold has different values to the same individual depending solely on how many oz of gold they already own. Hence, value is clearly not something intrinsic to gold, dollars, Bitcoin, or Steem.
Myth of Quantity Theory of Value
This theory says that as the supply increases value must fall a proportional amount. This oversimplification does not align with real world observations in the change in the supply of dollars. The dollar supply has increased by 4x with minimal inflation over the past 8 years.
Supply and demand dominate market price based solely on people looking to enter or exit a market today, not based upon the total quantity. If you double the quantity, but give it all to people who will not sell any time soon, then it has limited impact on today's price.
Myth of Revenue / Profits
As the founder of BitShares, I had a theory that transaction fees represent a form of income. Demand for transactions would generate fees which would generate a revenue stream from which value could be derived. Bitcoin has revenue from fees totalling less than $7 million per year, yet is valued 1000 times higher. Bitcoin has 0 profit. From this we can conclude that this method of valuing businesses is only a small and insignificant factor when the market perceives capital gains that far outstrip dividends. Gold and Silver are examples of assets that have value with negative rates of return due to their holding / security costs.
Social Value
Many people recognize that a dollar has no value to them, but they work for dollars because it has value to others. So long as someone, somewhere, values it others in turn will value it. This means that the network of individuals hom know and value a currency contribute to the value others will perceive.
Why will People buy Bitcoin?
So the question could become, "why will people buy Bitcoin?". After all for the first 8 years of its life it had an inflation rate that redistributed in excess of 10% of BTC to people doing work with little lasting value. If there is demand to buy Bitcoin then the same motivation to buy Steem must also exist.
The primary reason people buy Bitcoin is because it is a cryptocurrency with a large base of people who know about it and are willing to transact in it. The secondary reason is it has a large amount of liquidity. Steem has the potential to build a much larger user base than Bitcoin and provides financial incentives for liquidity. This combination means that Steem could become a better known currency than Bitcoin and thus become easier to transact in than Bitcoin. If market participants perceive this possibility they will buy.
Demand for Influence and Transactions
Anyone looking to transact on the Steem network needs Steem. There may not be visible transaction fees, but the demand to transact will drive people to buy and hold Steem.
Advertising Demand
This is similar to influence, but is geared toward an indirect monetary payoff. Anyone wanting to attract more attention to their content (aka ads in disguise) will buy Steem.
Monetary Use
At the end of the day, people will buy and sell Steem and establish a price history. Once a price history is established it is simply a question on whether or not STEEM makes a better money than other crypto-currencies. If it does make a better money, then people will buy Steem to use as money for transfers without any care about its use as influence on steemit.com.
Good money drives out bad money when no one is forced to overvalue bad money. Steem hopes to get into more users hands and therefore achieve a wider network effect. This in turn can make it better money than currencies with less network effect. Speculation on whether or not Steem will have better luck as a currency than other cryptocurrencies will be the primary driver for buyers early on.