There are plenty of reasons why people move. They can move for school reasons such as getting accepted into a program or to start a new career. They might move because they recently got married and their spouse is from a different city and they want to start their new life there. Those are all valid reasons one might move.
Whether the move is to a different city, province, state, country or continent, moving can be tough. There's a lot that goes into preparing yourself for a move. You have to research the new place you'll be living in, pack up all the items you can take, sell the ones you can't, update all your personal information with your new address, update your passport and get your finances in order, just to name a few.
If you're moving abroad, it can be even more work. As you'll have to get a work permit or visa, find a place to live in a foreign country, learn a new language, exchange your money for a foreign currency, etc. If you're moving because of a job transfer then your current employer might assist you with such things. Ideally they'll take care of your work permit or visa. They wouldn't be sending you to a different country to work if they knew you couldn't do so legally. They should also help you out with housing. Either find a place for you or give you some sort of relocation bonus.
Job transfers to foreign countries or continents aren't easy on you and your family, if you have one. So why bother moving half way across the world to work in a foreign market? Well, sometimes the opportunity is too good to pass up. Moving to a new country as part of a job transfer means that you're moving on up in the company ranks. That could lead to a promotion and perhaps a raise. At least you would hope so. You are moving abroad for the company.
Moving to a new continent because of a promotion or special job opportunity is also done to experience the world. Who doesn't spend their days dreaming about traveling across the world? Taking in one culture after another. That's usually a dream that you either complete by the time you finish college or university or give up on it completely. However, job transfers abroad provide you with that opportunity to do some world traveling. While at the same time getting paid and improving your current job skills. What's not to like about that?