"Here are a set of numbers that you can own and rigorously control (i.e. establish ownership). You can possibly exchange them as value, like a currency, but not many people will do that. Start the bidding!"
It's the most pure form of speculation imaginable.
Most other forms of speculation are derived, yes, from what we think 'others' will pay, but what 'others' will pay is usually derived from some economic value: i.e. a home is a place to live, a building a place to work, shares are rights to future profits, bonds get 'repaid' and pay 'coupons' ... so you can try to figure out 'what they are worth'.
BTC ... it's literally just worth what some other guy will pay - and that's 100% of the story. That's a difficult thing to predict :)