It is important to note that Shakespeare has the men discuss this good King to parallel Malcolm and Duncan with this good King and at the same time contrast the evils of Macbeth. In this scene, the purpose is to make the audience that much more certain that Macbeth is the evil character and that his country is diseased simply because he is on the throne. They need the help of the good King of England who has allied himself with the good future King of Scotland. Together, they can not fail against the epitome of evil embodied within Macbeth.
In addition, Shakespeare would be doing this to please the current King of England, James I, for whom the play was written. Being the King of England, it is good to know that Kings of England are the good guys, and also that King James' ancestry is Scottish. His ancestor, supposedly Banquo, was also the good guy in the story...killed by the evil Macbeth, but promised many Kings from his issue to sit on the throne.
So, the good vs. evil theme is supreme here, but there is also some very successful ego stroking going on.
They discuss the King of England, Edward, because he is thought to be able to cure disease with just his touch. This is an important asset for a king as it shows that he has powers bestowed from God thus is above ordinary people.
In this scene, all those who are "All swoln and ulcerous, pitiful to the eye" (4.3.173) visit the King for his benediction, his healing prayer and touch. The evil, is called the king's evil as he is the only one who can heal it. At the very end of the scene, they mention the King one more time as they have petitioned him for help and he has promised to provide troops to help them overthrow Macbeth in Scotland.