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.
I assumed that when your boarding card is scanned at security, it advises the airline that you have actually arrived at the airport and "checked in" - ie the on-line is actually printing your ticket, and the security area is the new "check-in" if you see what I mean.