Biden couldn't get the nomination for President despite repeated tries, but now as the sitting VP he is positioned very well, especially if he can avoid having to spend a lot of energy and money on the campaign trail and get the nomination at the convention.
Biden’s ‘I’m not running’ speech was the most “I’ll be ready if you need me” speech anyone has ever given. Agreed he’d be more dangerous than any others, but the anti-establishment sentiment won’t wear well on him, either.