They are only last leg of 3 leg flights (all with the same flight number) that travel YQT-YAM-YSB-YYB. Most people are likely flying from either Thundery Bay or Sault Ste. Marie. I suspect none of those city pairs have enough demand to warrant a non stop flight of their own, but combining them all on one flight fills more seats.
Conversely, YQR and YXE (serving much larger cities) probably both have direct flights to most of the same destinations, so linking the two doesn't add much value in most cases (the SunWing flight
@Urban Sky mentioned being the exception).