Why did they use periscopes?

Periscopes were used primarily in submarines to allow crew members to see above the water's surface while remaining submerged, providing a tactical advantage. They consist of a set of mirrors or prisms that redirect light, enabling observation without exposing the vessel. Additionally, periscopes can be employed in other contexts, such as in trench warfare, to observe enemy movements while keeping the observer safe. Their design allows for discreet surveillance and reconnaissance in various military applications.