They are filling seats that would likely otherwise be empty and which are being sold off cheap. The plane would be flying anyway. If a half empty plane was flying we wouldn't even have heard about it.
Having said that, it does seem a bit pointless. If these are so often available (see link below) the airline should reduce the flights to those destinations to encourage more full, not half empty, flights. I can't see that kids would get that much from a day trip to most places. I wouldn't have put my kids through the time and hassle just to spend a few hours in a city, however lovely a place.
Oslo, Venice, Tirana, Copenhagen, Milan, Luxembourg, Bucharest, Barcelona, Palermo, Turin, Naples, Rome ... £30 return. Dublin in a day has been a thing for a while.
https://www.hotukdeals.com/search?q=Day+trip