Reason being is that if you have any personal, private, or business information on that computer it could be read by someone else.
Just like people who take furniture out of your trash people do the same thing with that computer you just threw out. If they have the know how, they can look at all your information that was on that computer.
If you know how to, take out the hard drive and smash it with a hammer a couple times to ensure that nobody can look at anything you have. Just deleting things on it will not make your information go away.
If you don’t know how to do that then call me up and I can take care of it for you .