The importance of a strong idea in a text is the fact that text is built around a key idea. All of the features used in writing a tragedy like rising action, climax, falling action etc. are shaped around the key idea which in King Lear I would say to be Selfishness and how it can and will lead to your downfall as a fatal flaw. An impact that a strong idea could have on a reader could be the catharsis that it gets out of the reader, the purge of emotion which could be any emotion so any idea. For example, The fact that Lear has a fatal flaw and is clearly ruining himself throughout the play, the audience feels catharsis for him and the direction his life is taking. it makes the reader not only feel for the character but also reflect on their own life and how many times that key idea has involved them. This could have a shallow or deep impact on the reader and this is why having a key idea in the text is important.
Shakespeare presents many thoughts in King Lear about Human nature, Justice, and nothing which all turn out to be very important ideas.