We are not told why Jesus does this. Perhaps it was so that the man would know Jesus is busy helping him. It certainly is not the case that Jesus has to perform some kind of ritual in order to heal someone; he can heal people who simply touch his robe (Mark 5:24–34) and he can heal the paralytic simply by speaking (Mark 2:1–12).
23 And he took the blind man by the hand and led him out of the village, and when he had spit on his eyes and laid his hands on him, he asked him, “Do you see anything?”