To better understand steem you need to think of it like a company. All steemians are stakeholder in that company, the more SP they have the more influence they have on the direction of the company. Everyone upvotes what they think is best to grow the company so they can increase the value of their own stake.
The real value comes from the userbase ( attention basically), the more users we have here the more advertisers will be interested in this platform. User promoted content is already a big thing here which brings real value to the platform ( people are paying big bucks to bots to have their content seen) but right now most of the value comes from speculation, the prospect that steem is going to be huge in the future. If this happens advertisers are going to pay a lot of steem to be visible on dtube, dlive, etc...which will increase the value of everyone's holdings on the platform.
Another thing that gives steem value is its utility, the fact that it is useful and power a whole ecosystem, it's the transactionnal value. Steem is the blockchain with the most transactions per day check blocktivity.info. If steem is MLM then the US dollar is also MLM. The difference with steem is that it is very transparent and we know exactly where the new issued money is going. Another thing that gives steem value is the technology behind it which is called blockchain, you can send any amount of money instantly for free half way accross the globe, this is very valuable.