The type of lens used for a photo can alter the story that the image tells in many ways. Using a 50 mm lens blurs the background more, instead focusing in on the subject that is being photographed. A 50 mm lens can also be used for shots that are too far away to photograph with a standard 18 mm lens.
The 50 mm lens is heavier and a lot bigger. It is great for portraits and street photography where you want the subject and facial expressions or details to be expressed.
The 18 mm lens is instead a great universal lens. It is lighter and smaller and can also be great for street photography if you want to capture the overall scene of the spot or the big picture. The depth of field can always change when shot in manual, however, but this lens usually works to keep each component of the image in focus.
Below are photos taken from the different angles but almost the exact same spots, just using a different lens and as different people would pass by.
50 mm lens
18 mm lens