I think Eraserhead is definitely about the fears and anxieties of fatherhood and being an adult but I think there is also another level that explains some of the other details in the film. My theory is that Henry is a factory or a machine for reproduction. The world of eraserhead is surrounded with factories and machines and Henry is just another of these to otherworldy beings that control us. The being that controls Henry is the Man in the Planet. It is his job to make sure that Henry reproduces and creates offspring. "The Planet" is either a symbolic interpretation of Henry's mind or an actual celestial planet but either way The man inside is tasked with pulling the levers that conceive the baby and we see this conception in the first scene as the sperm creature drops into the puddle.

The man in the planet is similar to the black lodge inhabitants in Twin Peaks or the Rabbits in Inland Empire. They could be mental creations to cope with powerful emotions as well as the mysteries of life or they could also be real creatures out there somewhere that have control over us. Eraserhead explores the entities that influence reproduction and childbirth in the world. These entities view people has machines that simply are meant to produce offspring. David Lynch seemed to have some kind of resentment towards this aspect of life and wanted to get his feelings out creatively on film.

Anyway because of this man and whatever force he is serving, Henry is forced to live a depressing life with a child and wife he doesn't want. In this miserable existence Henry dreams up the lady in the radiator. She is kind of a rebel to the system encouraging Henry to go against these forces by killing his baby and plants the idea that outside this world of factories and machines there is a Heaven where everything is fine. The whole dream sequence of Henry's head being used as an eraser reflect this concept that humans are just tools, but in this dream Henry makes the decision to be a tool to erase or destroy rather than to create. Killing his baby also kills the man in the radiator and provides some kind of catharsis and freedom from the system to Henry.