Why is there added, "he descended into hell"?

That in my greatest temptations, I may be assured, and wholly comfort myself in this, that my Lord Jesus Christ, by his inexpressible anguish, pains, terrors, and hellish agonies, in which he was plunged during all his sufferings, (a) but especially on the cross, has delivered me from the anguish and torments of hell. (b)

(a) Ps.18:5 The sorrows of hell compassed me about: the snares of death prevented me. Ps.18:6 In my distress I called upon the LORD, and cried unto my God: he heard my voice out of his temple, and my cry came before him, even into his ears. Ps.116:3 The sorrows of death compassed me, and the pains of hell gat hold upon me: I found trouble and sorrow. Matt.26:38 Then saith he unto them, My soul is exceeding sorrowful, even unto death: tarry ye here, and watch with me. Heb.5:7 Who in the days of his flesh, when he had offered up prayers and supplications with strong crying and tears unto him that was able to save him from death, and was heard in that he feared; Isa.53:10 Yet it pleased the LORD to bruise him; he hath put him to grief: when thou shalt make his soul an offering for sin, he shall see his seed, he shall prolong his days, and the pleasure of the LORD shall prosper in his hand. Matt.27:46 And about the ninth hour Jesus cried with a loud voice, saying, Eli, Eli, lama sabachthani? that is to say, My God, my God, why hast thou forsaken me? (b) Isa.53:5 But he was wounded for our transgressions, he was bruised for our iniquities: the chastisement of our peace was upon him; and with his stripes we are healed.