Its not a stunt — she doesn’t need to do this before the election. But at her inauguration.
Why?
1) It will undo efforts to remove her from office.
From Day One, they will try to impeach her. They will not let up on the e-mails, they will not let up on Benghazi, they will keep probing and threatening to tie her up in investigation instead of governing. Unless removing her from office makes Kaine the incumbent President when he would have been running anyway.
2) It will give her cover to govern.
Obama’s best governing has been this last year as he is freely commuting sentences, and acting on his best instincts. She will have the broadest possible brush with which to make deals (and that is not a bad thing). Even if Dems somehow have the House and Senate for two years, it gives her the opportunity to force Dems to action on any number of items, because she can take the blame.
3) It will give the next generation of Dems time to set the stage.
Kaine, Booker, Warren, all are in play in 2020 and this gives them time to set their campaigns. I’m presuming Kaine is the prohibitive favorite, but it gives the most possible time for the Dems to run a competitive field. Likely Republican front-runners are Pence (ugh), Ryan, and Haley plus Kruz. A more serious (low bar here) lot of contenders than 2016 — it would behoove Dems to test their possible candidates in the primary.
4) Allay health concerns.
Every time Hillary has a cold, the right will be invoking the 25th Amendment. This will significantly tamp down running her health up the flagpole — the only purpose of which would be to call into question her serving at age 73 vs a Paul Ryan or Nikki Haley if she were to run for re-election. We have the precedent of Reagan winning in 1984, but we also have the serious concerns about his state of mind the last two years of his presidency. Hillary as showed no signs of issues around her age, but it will be fodder.
5) Mission Accomplished
She will have been elected as the first woman President against the entire machinery of the rightwing of this country. She is in this regard a hero. She has 4 years to show America that the right is full of shit, that she is not dishonest or untrustworthy, and that she is in fact one of the more courageous and honest political figures of our time. By giving herself the cover of not running again, she can show her best (doing hard work, getting things done) and not her worst (politicking).
Drawbacks:
SCOTUS: If Dems control the Senate I think we would see Breyer then RBG allow Hillary to appoint their successor. Would Thomas or Kennedy hang on 4 more years (rumors Thomas wants to retire soon) if she were only serving one term?
Congress: If Dems control the House and Senate, she’ll have 1 year to get stuff done — if the Republicans somehow retain the Senate and House, all bets are off.