1. To see the world
Some people choose to work at sea because it gives them a chance to travel or because they like the way they can get responsible jobs relatively early in their careers. But for others, it may be the only way they can earn a wage to support themselves and their families.
2. New horizons
The classic reason for going to sea was to visit exotic foreign countries. Although the experience could easily be seeing the sea for the first time, rather than seeing the world!
Undoubtedly, seafaring broadened horizons in more ways than one. It meant you had to work with the rest of the crew, often drawn from very different circumstances. Then there were foreign officials and workers who had to be dealt with, requiring tact and respect for local customs.
Lots of seafarers did see the world. Many British seafarers have very fond memories of visiting ports in Australia, New Zealand and North America. Their ships would often spend a week or more in a port, long enough for them to strike up a friendship with the local people. For the locals, the ship and its crew gave an important point of contact with an overseas country, sometimes the one from which they had emigrated.
3. To taste early responsibility
Not many people in their early thirties will be given sole responsibility for something worth tens of millions of pounds. This can and does happen to the master of a ship. It is the master's responsibility to manage a crew that, in a big cruise ship, can number thousands. The master has to ensure the ship plus its cargo or passengers arrives safely and on time. There are many people to do the work, but the ultimate responsibility is the master`s alone.
It is not just the captain who takes responsibility when fairly young. A junior deck or engineering officer will take watches where he or she is in sole charge of the bridge or engine room. Of course, a superior can always be called. However, there is a strong incentive for the juniors to show they can solve a problem without help.