Layers are desigend to help you make and edit things. You can hide things in on layer to help you see another layer, and you can lock a layer to keep you from accidentally slecting or editing things in it.
For example, in a Scene if you put the interior of a house in one layer, the roof+ceiling in another, the furniture in a third, and your characters in a fourth, you can lock the house and furniture when you are working with the characters so it won't accidentally be moved, hide the roof+ceiling so you can see the whole scene from the top without them blocking your view, etc.
In the Object editor you can put different parts in different layers and lock/unlock, hide/show them with a single click to help see what you're modeling.