Why do I need to evaluate my sources? Because your paper is only as good as your sources.
Let's use a practical example to illustrate this. Pretend you are fixing up an old car, and you'd like to sell it for a small profit. A lot of parts in the car don't work, so you need to buy some to get it up and running. Can you go to the junkyard and just get any old parts? No, not if it you want it to run.
First, you need to have an understanding of how that particular car works, and then you need to be sure that you have good quality parts. You don't want the car to break down right after someone buys it. The car will only be as good as the parts that you put into it.
Writing a paper is like fixing up that car. You can't just use any old source that you find on the Internet or in the library's databases. First, you need to have an understanding of your topic or argument. Then you need to be sure that you pick sources that are not only appropriate to your thesis, but ones that are also of good quality. Like the car, you don't want your paper breaking down halfway through!