[blooalien]

It places related content or features of a program alongside or above one another in the same program window so you get all of what you need all right there on the one window. The separate spaces are often resizable, too, so you can fit the content that's important to you however you as the end-user want it. Thus why as another commenter here (izzieto) notes, it's used in almost any application you can think of.