I can't understand it myself either. Weather forecasts are a major part of pre flight and in flight checks I'd have thought. Surely if a pilot sees big problems arising at a particular airport e.g. Dublin tonight s/he would either delay take off to see if weather improves or cancel the flight after a few hours.
Surely it costs the airlines a fortune to keep the plane circling and diverting, and all that extra payload of fuel "just in case" they have to circle or divert, AND transfer passengers from the "wrong" airport to the right one overland. Oh I'm sure there are valid reasons, maybe the airline insists on it, maybe the pilots want to "have a go" I really don't know though. But it doesn't make much sense to me when they know things are pretty bad at the destination airport.