As far as I can remember, Otanashi wasn't even supposed to end up in the purgatory since he died having without regrets. He wanted to be able to help and save people. That was fulfillled when he signed his life insurance card to donate his organs, specifically his heart, to anyone who needed it. Kanade's life was saved but she died with the regret of not being able to thank the person who donated his/her heart.
It's never really been further explored but Otanashi seems to believe he was sent to the purgatory to help people who were [I]lost[/I] to pass on.
Oh and to highlight in black any text that may have spoilers like I did type[spoilers'][/spoilers] without the apostraphe and type the text in between.