Temporarily for now. (So he says!) So he can call a general election and get the Tories out, and get Labour in (so he can have the job permanently!) And maybe even derail Brexit. Even though he has been very anti-EU all his political life (40-odd years!) he seems to have fallen in love with the EU this past year or two! 
talkradio.co.uk/news/jeremy-corbyn-make-me-temporary-pm-stop-no-deal-brexit-19081531927
Personally, I actually laughed out loud when I saw the news this morning, and saw that he wants Westminster to vote him in as Prime Minister. The man is so deluded, and off the planet, that I'm not sure he should be leaving the house, or holding a hot kettle, or in charge of sharp objects, let alone running the fecking country! 
YABU if you disagree with me.
YANBU if you AGREE with me.
You probably already know that.
I'll get me coat....... 