Chances are that if someone visits a bad address on your website (it could be a mis-spelled URL, a page that’s since been deleted, etc), they’ll see a 404 page.
It’s simply a catchall page that visitors are directed to when a page URL is invalid. And most webmasters do nothing with this page.
However, it is a page that can be monetized. Why not put an ad on the page for some product related to your niche? You likely won’t get many sales, but you’ll get zero sales from a standard 404 page.