First, even though Porfiry is able to explain so much, the reader must step back and acknowledge that the information that Rodion Romanovitch is the murderer is still only circumstantial evidence and in modern courts would be insufficient to bring about a conviction.
Porfiry's explanation of the crime and his refusal to arrest Raskolnikov show that he does sincerely like Raskolnikov, but more importantly he also believes in Raskolnikov's greatness. Porfiry's true purpose and mission becomes clear in this chapter. First, one must understand that Porfiry, like Dostoevsky, was a dedicated Slavophil, one who believes that the Slavic people are a type of "chosen people." In other words, Porfiry believed so strongly in the greatness of Russia that he is constantly searching and helping those who he thinks will be the future leaders of Russia or who will be able to contribute to Russia's greatness in other ways. Therefore, he views Raskolnikov as a man of noble character, one of the young intellects of Russia who could be of great service to the state if he learns to reject his radical ideas. Porfiry attempts to force Raskolnikov to acknowledge that his theory is wrong, and from this confession to go on and face life and become one of the most important minds of Russia. If Porfiry were to arrest Raskolnikov immediately, it would ruin Raskolnikov's intellectual redemption through self-realization. But if Porfiry gives Raskolnikov enough time to confess on his own (and thus realize and acknowledge to himself his own error), then Raskolnikov will achieve a greatness in his own right. Therefore, it would be no advantage to arrest Raskolnikov unless it is for simple punishment, and Porfiry has greater things in mind for Raskolnikov than punishment; he wants redemption and greatness from Rodya.
Raskolnikov's confession earlier to Sonya represents one aspect of his character and Sonya is trying to redeem Raskolnikov by asking him to take up his cross and suffer. As a parallel, Porfiry also emphasizes the importance of suffering, which accounts for Nikolay's confession, but Porfiry emphasizes the importance of suffering as a means of expiation, "for suffering, Rodion Romanovitch, is a great thing."
