The better, more selfless reason is that they believe in its decentralized ideas and might not fully like a centralized system that most of our financial systems have so far, and they like what blockchain technology is doing to our society and therefore are investing.
I don't want a centralized system too like our current financial system that we are using right now that is why I'm supporting Bitcoin which is a decentralized currency. I believe that more people will support Bitcoin in the future.
The other reason is purely money-based and they just get the idea from friends or media outlets that BTC will constantly go up as a currency, and in a year they can make thousands of the coin - I think this is the more popular reason nowadays.
Trust. They believe that in the next years, Bitcoin will give them profit that is why they are buying and holding Bitcoin.