The Dreaded ";"

Why in the world do we have a Semi-colon? For the most part I don't know how to use it and think it is unnecissary. Well It definately helps for when you are listing people and their profession like, "This is Mike Marigold, the butler; Janice Jory, the maid; and Mr. Body, the owner. You are I think you are supposed to use it when combining like ideas, too. Such as, "Bill is going to the store; however, he has to do his homework first. HERE Gives a good example of these things.