Traditionally, periscopes offered the submerged submarine its only glimpse of the outside world. Movies have also made them the submarine's most familiar feature.
Periscopes are still useful, despite the sonar and electronic sensors of modern submarines.
They can also be fitted with video cameras or other means of collecting permanent data. But operating at periscope depth, just beneath the surface, submarines are relatively easy to detect. To prevent detection, periscopes are used as infrequently and briefly as possible.