I know many people who used to buy multiple tickets for the same journey when <10€ fares really were readily available, and in the airports I frequent, it was common to hear Ryanair gate staff calling for a final few passengers who never showed up. Those flights were never late taking off (unless for other reasons) but many could have had their doors closed as much as 15 minutes earlier if the phantom checked-in passengers had not been on the list.
What intrigues me about advance check-in is the declaration that nobody has interefered with one's luggage when chances are the suitcase hasn't even been dragged out of its habitual wardrobe or attic resting place.
