Business travelers in particular often find it cheaper to book multiple low-cost non-refundable tickets than buy a flexible ticket with a major carrier.
Used to happen quite a lot on buzz - the acceptable face of LoCo flying - between STN and FRA. That's the real Frankfurt, not some ex-USAF base hours away in the Moselle.
A great little airline, before the wooden-headed, wooden-footed people at KLM sold it to the odious O'Leary.