For example, if my email is firstname.lastname@example.org, then I would use email@example.com when signing up for Hacker News.
Sometimes, I don't actually remember and just end up going to my inbox to find an email from the site so I can look up what I used.
Another time, this backfired on me when I purchased concert tickets using a +descriptor email and couldn't login. The purchase form and login form had different validation rules, so the purchase form accepted my +descriptor email and charged me for it. The login form rejected my +descriptor email and I was rushing to contact customer service to print my ticket in time.