how come wrist watches replaced pocket watches? When did the change started to happen?
Although wristwatches had been around from as early as the 17th century, they did not become widely popular until the very end of the 19th century, largely due to a change in military tactics in European armies.
The British army began to use watches to synchronize attacks with separate groups across large areas, but it was found that pocket watches were too cumbersome and impractical to use in the heat of battle, particularly for cavalry troops, and so wrist watches were adopted. Wristwatches then gradually became a common aspect of the British uniform and were widely used by almost every British soldier.
Over time, these soldiers then began to use these watches in their civilian life and they gradually became commonplace within European society. By the end of WWI, a conflict which almost every male civilian in Europe and America was involved in, wristwatches had become the standard form of timekeeper within Western society.