How does subplot help the main plot in King Lear?

Shakespeare's intent is to show how one generation gradually supplants the generation that bore and nurtured it. The subplot deals with meles, whereas the main plot deals with females. This suggests a universality to the theme. Keats speaks of "hungry generations" treading down the older generations in his "Ode to a Nightingale." This is really what King Lear is about. The characters and their stories are only rather extreme examples of a general rule. 

Furthermore, the subplot is really necessary because the main plot has more or less stalled. Nothing is happening. King Lear disowns both his daughters and goes out into the open country to live like a vagabond. His daughters make no effort to persuade him to return to the shelter they are willing to provide if he accedes to their terms. There is a standoff in the conflict between Lear and his daughters. This is dramatically dangerous. It risks losing audience involvement. This is where the subplot involving Gloucester and his two sons is needed. Edmund wants to take over everything his father owns, including his title. He writes his own sentiments in a letter but attributes them to his naive brother Edgar.

'This policy and reverence of age makes the world bitter to the
best of our times; keeps our fortunes from us till our oldness
cannot relish them. I begin to find an idle and fond bondage in the
oppression of aged tyranny; who sways, not as it hath power, but as it is suffered. Come to me, that of this I may speak more. If our
father would sleep till I waked him, you should half his revenue for
ever, and live the beloved of your brother.             I.2

Edmund's treatment of his father parallels what Goneril and Regan have done to Lear. Soon Gloucester will find himself out in the cold, just like Lear. There will be a very poignant meeting between these two old men in Act 4, Scene 6. It is both sad and funny. Life is a tragedy. Life is a comedy. It always has been. Like a lot of old men, Lear and Gloucester discuss the human condition from the point of view of old age.

Only the man who attains old age acquires a complete and consistent mental picture of life; for he views it in its entirety and its natural course, yet in particular he sees it not merely from the point of entry, as do others, but also from that of departure. In this way, he fully perceives especially its utter vanity, whereas others are still always involved in the erroneous idea that everything may come right in the end.
                                                                  Schopenhauer

Lear and Gloucester have lost everything. They have been pushed aside. Nobody cares about them. So they are both philosophical about life. They do not talk about their offspring, because their children are only examples of a general rule. They are more interested in the general rule itself. They talk about humanity as a whole and about the meaninglessness of life. This is where Shakespeare states his thesis, which is the guiding light of the whole play.