Symbolism, Imagery, Allegory
Robot Primus and Robot Helena don't have much to do as characters. They only show up at the tail end of the play, and they don't do a ton except fall in love and then talk about falling in love. Then they go forth to get married, maybe taking the formula of life with them so they can make robot babies if they want.
So, these two are not very exciting as people (or, well, robots). But as symbols, they've got a little more to say. Primus and Helena are directly linked to Adam and Eve—the ever-flowery Alquist calls them Adam and Eve directly: "Go, Adam. Go, Eve" (3.228).
Alquist then quotes Biblical verses describing the creation of humankind. You don't get much more explicit than that. Primus and Helena are the mother and father of humanity; they're symbolically the first man and woman on earth.
They're also, though, the children of humanity. They were created by Domin, by the Rossums, by Gall (who gave them souls), and by Helena (who told Gall to give them souls). If Helena and Primus are Eve and Adam, that would make human beings God, or at least god-like. The play ends with Alquist talking about love and spirituality. But you could also see it as ending with victory for Old Rossum, who wanted to prove that God didn't exist by creating a human being himself.
Primus and Helena seem like they're supposed to symbolize the triumph of love and spirituality. But they could also mean that, hey, you don't need spirituality, because people can put together a human being without bothering with all that spirit stuff. When robots turn into humans, is that the triumph of humanity, or the triumph of the machine?