More Touches on the Ball - When playing indoor, in a normal scenario, the setter will get 33% of the touches.  The other 67% of the touches are split among the 5 other players on the court. The score may be 5-3 and a player may not even have touched the ball once. When you play doubles, you are getting an average of 50% of the touches. This alone will speed up your improvement.  

Becoming a Complete Player - When playing indoor, depending on the position you play, you may only learn and perform a few skills. When playing doubles, you are forced to perform all the skills that the game has to offer. You become a complete player, making you more marketable to coaches and multiple positions at any level. 

Self-Coaching - When playing indoor, you always have a coach telling you what to do, where to go, etc.  When you go to an outdoor tournament, it's just you and your partner coaching each other, which helps you work through the process of strategy and problem solving as a team which will enhance your volleyball IQ

Toughness - When playing indoor with 6 people on the court, unless you're the setter, you can hide flaws and cover up bad games.  With outdoor, it's just you and your partner which means there is no bailing you out.  It's a sink or swim game which forces athletes to grow up and become mentally tough.

Athleticism and Endurance - Training in the sand creates resistances which will recruit more muscle, helping athletes become faster and jump higher. It's also a heck of a workout and will get you in shape. 

The Atmosphere - The outdoor game is the more laid back version of indoor volleyball.  Listening to music, wearing shades and playing in bare feet set the atmosphere.  Vibes are good and tranquility is key. 

College - Beach Volleyball is now an NCAA college sport.  Imagine playing the sport you love on beautiful beaches across the US.

The Outdoor Community - The outdoor volleyball community is second to none and there are so many tournament opportunities for the girls to play as they graduate high school and college. Outdoor doubles is something the girls can continue to do for a long time and you only need to find 3 other friends to train with, which makes it a lot easier as you become an adult.