For non-overseas flights, wearing the suit on board means you don't have to go through the hassle of packing a suit. Packing a suit will generally mean (and yeah I know there are exceptions) checking bags. If you can avoid it by wearing a suit, why wouldn't you?

Going overseas, I suspect it is more a matter of choice. Some people like to look polished and presentable at all times. I don't even think there is any kind of sacrifice of comfort... a well tailored suit is very comfortable to wear. I guess not everyone rushes out to put on a t-shirt and jeans at the first possible opportunity.