You may or may not need them later on but they do take up space on your system drive.
They’re also created when you try to extract an archive, as the archive first gets extracted to a safe temporary location and then gets copied to where you originally extracted it. Once you’ve closed the browser, some portion of all these downloaded files remain on your PC as cache to give your instant access to your favorite websites and videos. The same thing happens while watching YouTube videos, your browser cleverly downloads the entire video in chunks and plays it. When you browse the internet, your browser essentially downloads heaps of data from the internet to render webpages and their images.
