Our eNotes site on Lear provides an excellent and direct answer to your question. Referring to a literary critic, Ian House, it begins "House emphasizes the dynamic relation between the main plot and the subplot in King Lear, proposing that the differences as well as the similarities between them unsettle and illuminate our understanding of the principal story. As the critic explains, the double plot universalizes the action by shifting emphasis away from individual characters and situations; the effect is more like that of a prism than a mirror, multiplying images rather than giving back a single one. Further, House analyzes the notorious implausibility of dramatic events in Lear, arguing that the absurdity is purposeful and heightened by the changes in the humorous tone of the subplot "from farce to melodrama, from domestic tragedy to surrealism." In the course of discussing these issues, the critic provides extended evaluations of Gloucester, Edmund, and, especially, Edgar.] click on the url below for the full discussion.
Traditionally, critics saw the events of the subplot to be repetitious of the main plot. The subplot deals with the tragedies of Gloucester while the main plot deals with Lear's problems. More recently, critics have been looking at the differences between the two plots. Most modern critics see the subplot as serving to intensify the main themes of the play, using it to further demonstrate such themes as the ingratitude of children and the idea of individual identity.
What does the above mean? Not all the critics agree as to the purpose of the subplot. I would suggest looking at the differences and similiarities of the two plots and drawing your own conclusions. Look at the central themes of the play and see how the events of each plot demonstrate those themes. When you get a question like this, where you don't have a clear answer, always cite evidence from the literature to back up your belief, and most teachers will accept your answer. I hope this helps in answering your questions.