Why did sailors have a flap on back of trousers?

Sailors had a flap on the back of their trousers, known as a "foul weather flap" or "sailor's flap," primarily for practical reasons. This design allowed for ease of movement and comfort while working on deck, especially during harsh weather. Additionally, it facilitated quick access for personal hygiene needs without having to remove the trousers entirely, which was particularly useful in the cramped conditions aboard ships.