Mainly because they believe it will stay valuable. Which is what backs all stores of value - gold and diamonds are definitely not bought for their practical applications.

Now why exactly people believe it is and will stay a good store of value is more complicated, but mostly a mix of trust in how miner self interest will keep the keys safe and general desirability of online currency

Because some other nerds got really rich on Internet fun bucks and other people want in on it too.

shhhh you'll burst the bubble