Why does Roy save Deckard?
Roy Baty has has ended his rebellion against humans and given up fighting against his fate of a too short life (how human). Intellectually, he knows his time is up-his body is failing. More important, his spirit and will to fight is dead. It died with Pris. Without Pris or other replicants he would be relegated to being a talking machine. Without meaning there is no life. He and his friends have failed in their quest, so he lets go.
Even though he is a machine, Roy is 'human'. This is stated in two ways. The rain soliliquy is the same summary any human might give at a moment of utter defeat. Also, saving Dekard's life is Roy's statement of awareness of shared humanity and the value of life. Roy is a dead man walking (or sitting). Depriving Dekard of his life serves Roy no useful purpose. As a statement, it is even more powerful because he sees what biological humans are too prejudiced to see.