You or your mum might have an old computer that still works but neither of you have any use for. Unless the computer contains nuclear launch codes or bitcoin mining data, you may just want to delete all content from the hard disk to ensure any old documents or personal information don’t stay on the disc if you decide to sell or give it away.
Even if you were to take it to a recycling location you probably still want to clean the hard drive of any data. This doesn’t mean you need to take the drive out and smash it with a sledgehammer, but you do want to use a software erasure tool that will stop the majority of people being able to unerase the data that you have deleted from the hard drive.