Do you mean their private cryptographic key?

Besides being a statisical feat along the lines of being struck by lightning every day for a year while also winning the lottery every day, whomever got that key would potentially be obscenely rich. However, spending the money would be a challenge.

That said they'd have to be very very careful. Hopefully the only kind of person that would find that key would have the additional good sense of security and paranoia that Satoshi also had. They'd keep their mouth shut and work hard to keep the secret of their discovery to themselves.

If I had that I'd break it up into a few tens of thousands of smaller wallets and then transfer those directly to different exchanges before chashing out. Or maybe try to find a big company to sell them to.