There are various reasons to choose SNG tournaments over ring games. Perhaps the most important advantage of tournaments is that they may, in many respects, be considered a science. Their structure allows the player to follow a relatively precise formula to defeat them. Each SitNGo has the same number of players and follows a structure that remains constant tournament to tournament. As such, most poker variables remain constant and predictable. On the other hand, ring games contain a great deal more variability. Your play requires constant variations based upon the climate of the table. Players are free to come and go as they please in ring games and may buy-in with more funds whenever they like. In tournaments you are playing with the same players from beginning to the end, so you begin to understand how they play. It's much easier, and more profitable, to teach someone how to be a professional SNG player than a professional ring game player.
To be a professional-level ring game player, you must be willing to play at a high limit. It can be very challenging and frustrating to build your bankroll up to the point where you can safely play the higher limits. Contrary to this, SNGs provide you with a smoother progression, allowing you to both learn and earn as you go. The most crucial reason is the fact that SNGs have less bankroll variance than ring games. This means that the financial downswings (a mathematical part of the game) will never be as large as in ring games, so the stress imposed on the player is reduced, and learning is much more enjoyable.