Say the editors of The New York Post.
I've got another answer to "What's in it for her?": She can antagonize Donald Trump from that position.
The NYC mayoral race is this year. The incumbent, Bill de Blasio, is eligible to run and has said he's running. Wikipedia has a nice list of declared and potential candidates. I'll just highlight the ones that got my attention. From the potential Democrats, in addition to Hillary:
Anthony Weiner, former U.S. Representative for New York's 9th congressional district and candidate for Mayor in 2005 and 2013Yes! What could go wrong? There's no further depth of humiliation. Anthony, anything you do will feel like redemption. Say yes!
From the potential Republicans:
Donald Trump Jr, businessman and son of President-elect of the United States Donald J. TrumpOh, yes! The time is ripe. Building a dynasty is not something you drag your heels on. And what if the Democratic nominee is Hillary? The Trump vs. Hillary show was a big hit last fall. I want to watch another season. Make it happen, people. It's a slow year for election action. The spotlight is on you. Bill de Blasio richly deserves a big challenge. Come on, New York City. We political spectators need some variety. The Washington show is worth watching: President Trump, Trump's Congress, the Democratic Party in Trumpland. But all that governing can get tiresome. A nice active NYC mayoral campaign would be great.
And I'm saying that as someone who reacted to the 2016 election by saying at least Hillary will go away.