I don't think it is ihe primary purpose of cryptocoin either. It's just that crime found a great valuable tool in it. Just like nuclear fission is not just for atomic bombs. Or the internet for porn. The most known use case is not always the only or intended one.
Not defending the money-grabbing, resource-hungry spectacle that it has become, but the original intent was probably a bit nobler than the current results.
ETA: ah, I see what you mean. Purpose as in current main use, not "intended purpose". I think I picked up the wrong meaning. My mistake.
Also, nothing wrong with porn, but everyone knows the internet should be about cat pictures.