You've already worked a little with GIMP's simplest selection tools: Rectangle, Ellipse, and Free (Lasso) Select.
So far, you've used selections primarily for drawing. But selections are more commonly used to choose one part of an image and ignore the rest.
It comes up in all kinds of situations. Perhaps you have a photo of a person against a complicated background, and you want to make the person stand out more, or paste a portrait onto a simpler background. Or you might want the opposite of that: to erase someone or something from a photo.
You may want to change or enhance part of an image—remember how the Bucket Fill tool could change the sky but not the rest of the photo in Chapter 4?
Sometimes it's just a simple touch up. ...