Because it makes research for school so much easier. It's a lot easier to settle on a source when you can write down the subjects you'll be using in your outline, and then after searching for sources for one subject or subtopic, search again with another subject/subtopic. You can note down which sources show multiple times. (i.e., they have the info you need for more than one subject/subtopic) which is a lot easier than having 15 subtopic and 17 sources.Much easier to have 15 subtopic and 6 or 7 sources,or at least that's how I do it.
But I am not, nor have I ever been, will be, or have the desire to be a scholar.