Let me suggest a hypothetical example where the "reward" is simply the ability to
send an encrypted message (via the block chain) to another account.
The bitcoin is also the 'ability' to append some 'message' to a public file named 'blockchain'
Either you can trade this 'ability' to money or it is useless for you and everyone else.
I'm sorry but just because you say something is "useless" actually doesn't make it that (perhaps you have too high an opinion of your own opinions and if not then please stop making such unhelpful assertions about what is or is not useful to people other than yourself).
Some people may actually value the ability to send a private message to another person (and you can't actually trade that as you would be having to trust someone else with your private message then).
Thus the ability to send a secure message via a blockchain could very well be something of value but not something that can be traded (of course I'm sure you won't accept this reasoning as you are not reasonable but I think some others may be able to see the logic).