Get Out of Free Card
Disclaimer: I don't own Charmed!
She wanted to know exactly what he was getting for helping her. But Cole really couldn't tell Piper the truth. He wanted to see Phoebe just one more time before he leaves. He wasn't going to get out of the plane he was trapped in and go to heaven. To Cole it was ironic how he would get to go to heaven, but his human side did a lot of good, and after all he was misguided, not truly evil.
He wanted to see Phoebe again, but instead he would get out of free card in this plane. But he wanted to make sure that Phoebe would love again. And what greater love can she possible see if she saw Piper's and Leo's love.
And that he told to Piper. She needed to die, in order to save Leo.
It was the only way he could get out of his personal hell and to see Phoebe without her seeing him again.