What is the reason for Shakespeare to introduce a subplot （the story of Gloucester )？
Although we can do a critical analysis of how the sub-plot functions in the pay, in thinking about this, we should also be aware that Shakespeare was a professional playwright whose job was producing texts for a specific company, and so practical factors such as providing good roles for certain actors, pleasing certain patrons, filling a certain amount of time or reusing existing sets and costumes might well have factored in to his choices.
First, double action plots are actually quite common in theatre (and films and TV); the ability to switch back and forth, especially leaving cliffhangers, can generate suspense.
Second, the Earl of Gloucester resembles Lear in many ways, and serves to illuminate Lear`s characters and choices, as well as the theme of filial loyalty.
Finally, the sub-plot gives us a much stronger sense of the characters of Edmund and Edgar.