It's a call to action. If you see that 18 people are looking at that route and there's 1 flight that's really cheap then it's an incentive to quickly book that

It's a call to action. If you see that 18 people are looking at that route and there's 1 flight that's really cheap then it's an incentive to quickly book that flight before the last seat is sold out. It's also the reason why other OTAs will list sold out hotel rooms or say "2 seats left" for a flight and why Amazon will put "3 left in stock" for certain items.