Greed? Fraud?
I could understand a hard-line Democrat activist taking and releasing the data: just give it to CBS, the DNC, whatever, it gets out, it (presumably) hurts the object of their hate, job done.
It might be a hot news scoop, if there's something important in there - any reputable media outlet would protect their source's identity (remember Deep Throat?) and I'm sure a few would pay for a good exclusive. Again, story gets out, source gets some money, job done.
Offering it to the Republicans too, presumably to bury if there is anything secret, is much riskier for little reward: probably crossing the line into extortion - indeed, from the story they've already involved the police. Just leaking the data, even if caught, you might get away with it (Richard Armitage was never charged for naming Plame as the committee member who sent her husband to Niger, despite the big investigation that sparked) - but blackmailing? There's a cell for that.
Bitcoin would protect the recipient, but whoever pays up would be known. If the Republicans paid up $1m to bury the "tax returns", wouldn't that be a huge incriminating story in itself? If they had the real ones, no doubt they could quote something that would confirm the authenticity: a serial number, the exact value of some specific deduction claimed, something like that. PwC's denial (assuming they don't actually mean "missing"; I'd imagine their IT people checked and there hadn't been any access to those files) seems to imply there's nothing at all to verify yet.