Some things aren't available for streaming, so you have to find a way to download it. Sometimes you have a bunch of DVDs you want to store and watch digitally without buying it again.
And then sometimes there's shows that just aren't worth paying Amazon $25 each for 15 seasons so you pirate it.
Now you have video on a disk somewhere and want to watch it on your TV and phone and tablet and grandma's computer. Systems like this are arguably the least painful way of doing that.
Will you watch any of those videos multiple times? Probably not, but it will always be there if you want it. It won't randomly disappear on the whims of a third party just when you want to show a new friend this cool show they definitely need to watch.