A flask is traditionally given as a gift to the groomsman on their wedding day, but is also very commonly used by men who venture outdoors on camping trips and to do other adventurous activities.
Typically, a flask is reserved for the man’s alcoholic drink of choice which is more often than not a strong whisky.
The reason for this is primarily to stay warm on bitterly cold nights outdoors, but it’s also a fun way to get a bit of Dutch courage and help you through uncomfortable situations.