A common reason is that users often like to offer the first episode of a series free as a way to entice people to subscribe to their platform.
In this case, users would make the avails for that episode FVOD (Free Video on Demand) and SVOD (Subscription Video on Demand). This way it's available to the general public as well as those who have a subscription.
Users may mix and match avail subtypes at their discretion to create unique offerings.
To learn more about avails, select this link