There are countless sites that highlight reasons for teaching abroad. 
 
At TEFL Source we believe the most important reasons are opportunity and adventure.  Outside of travelling and volunteering, there are few opportunities in the world that allow people with normal skills sets (people who are not engineers, scientists or doctors) to live and work abroad for extended periods of time. The TEFL / TESOL industry offers this 'live abroad' lifestyle to the masses and the best part is the opportunities are readily available in every region of the world with benefits that continue to evolve with the rapidly growing job market.  
 

“It’s better to see (experience) something once
than to hear about it a thousand times”

Why are there so many English teaching jobs?  There are two primary reasons.  The first is because English is the only true globally recognized language.  The United Nations, World Trade Organization (WTO), World Health Organization (WHO), multinational corporations, the international scientific community, just to name a few, all use English as their primary means of communicating.  Therefore, it’s important for students and workers who live in non-English speaking countries to study and learn English in order to remain competitive.  Because of this factor many developed and developing countries have robust EFL industries (English classes in public schools, private English academies, private English tutors and business/corporate training) that employ native English speaking teachers and instructors.
 

“Regretting things you did is far better
than regretting things you didn't do”

The second factor is job turnover rates.  Have you ever noticed that many coffee shops and fast food restaurants constantly have a ‘Now Hiring’ sign hanging in the front window?  As you can guess, this is because most coffee shop employees are only working there short term - until they find something better.  Therefore, management is constantly hiring and training new staff because the jobs they provide are, for the most part, a revolving door (new people constantly coming in and old ones constantly leaving). The international English teaching industry experiences the same obstacles but in a different way.  Most people have no intentions of moving to China or Brazil for 20 years in order to teach English.  In fact it’s quite the opposite, most people want to go for the ‘contract term’ only then head off to another country or back home to see family and friends again.  The point is, jobs are constantly available because international English teachers are always on the move (revolving door).
  

“6 months abroad will teach you 
more about yourself than 6 years 
at home ever could”