Earlier, Porfiry's technique was called a "cat and mouse game." This now becomes clear to both the reader and to Raskolnikov. First, Raskolnikov is kept waiting unnecessarily; then he is exposed to incessant chatter and more chatter; and then there is the hint of some secret evidence hidden in the next room. In all these cases, Porfiry does have the upper hand, and Raskolnikov is at his mercy.
The meetings in the last two chapters show us that Raskolnikov goes from Sonya who will redeem him emotionally through her sufferings to Porfiry who will try to redeem Raskolnikov intellectually. This second redemption is more difficult since Raskolnikov's existence now is based upon the validity of his theory.
Porfiry presumably knows more than he reveals. He could arrest Raskolnikov at any time, but doesn't because he does have a "sincere liking for him" and if he arrested Raskolnikov now, Raskolnikov would never realize the error of his theory. Religiously, confession of sin is the beginning of redemption and Raskolnikov must be left alone to confess.
