You might find that a lot of people sign up for a free webinar and either miss it, or want to re-watch it. Or, they might come across it after the event. In these cases, you can sell single replays of your webinar. If they buy once, they may very well buy again, so this single access allows you to collect their email addresses, purchasing history etc. The more data you have about a customer, the more you can anticipate their needs.
This strategy works really well if your content is valuable and less sales focused.