Scuba diving for professionals needs no explanation – it is their job to dive. Why they dive though is a different story. Some dive because they are part of scientific teams exploring deep sea life, others help salvage wreckages and deal with public sanitation requirements. Then there are forensic divers responsible for finding out causes of mishap for airplane and ship accidents.
From a recreational point of view though, scuba diving can translate to numerous interesting underwater adventures such as ice diving, deep diving, cave diving and wreck diving. And if you truly fall in love with scuba diving then you can dive just to instruct!
For divers who work, they can be part of recreational community activities or full time work. Recreational work includes dive masters, dive guides, instructors, assistant instructors and so on. National laws and regulations decide who can be an instructor and so on.
In the military, diving is more objective. They are called frogmen in military and their roles can vary. It can include infiltration behind enemy lines, direct combat, bomb disposal, placing mines or simple engineering activities. Police using diving teams for search and rescue, recovery and detection of crime in water bodies. Even lifeguards are trained divers should the need arise.
Finally, there are professional divers who are there for the purpose of photographing underwater life or studying them. Students of marine biology, hydrology, underwater archaeology, oceanology and geology tend to learn diving too. Not to forget avid photographers and videographers.