Why do cabin crew also man check-in desks? When flying, I sometimes notice that the person behind the check-in desk also turns out to be the flight attendant on my flight. 


Is this always the case, or did I just notice the exceptions? 
Why do airports use cabin crew for the (comparatively) simple job of checking bags, which could in theory be done by (presumably) cheaper lower-skilled employees? Or am I underestimating the difficulty of checking bags?
It probably depends on the airline and airport, because on the flights I have been on, the checking-in is managed by "check-in staff" who are not flight attendants. Larger airlines do the arrangement that I just describe while smaller ones might manage everything with the flight crew.