In his soliloquy Edgar explains that he realizes that a price is on his head and that he has decided to take upon himself the disguise of Poor Tom, the Bedlamite in order to escape capture.
His final words are important
Poor Turlygod! poor Tom! That's something yet: Edgar I nothing am.
This "something" from "nothing" is in contradiction to Lear's "Nothing can come from nothing."
As Cordelia had been reduced to "nothing" by her father but turned herself into her father's Fool, so Edgar has become "nothing" and yet has turned himself into "something" - the Bedlamite, Poor Tom. This scene in the sub-plot confirms the action in the main plot.