A membership area is where the digital product is actually delivered: lessons, PDFs, community, files. It exists to separate who has paid from who hasn't, and so that access doesn't depend on someone sending an email by hand.
When the area is integrated with the payment platform, the sale's approval triggers the creation of access within seconds, and cancellation or refund removes that access the same way. When it isn't integrated, someone has to do both things manually, and that's where most complaints about undelivered products come from.
Fast, verifiable delivery is also a defense against chargeback: the record that the buyer accessed the content is evidence in the dispute.