Motivation. There really does need to be a significant why to motivate us to change behavior. And that why needs to be even stronger to encourage us to keep at it once our initial goal is met (losing 10 lbs, fitting into the dress, finishing a barre challenge).
Studies say that commitment success can be analyzed through comparing "want to" vs "have to" styles. For example, you go to a yoga studio you've been curious to check out. You surprise yourself with how much you enjoy it. You're not staring at the clock. The women in the studio seem welcoming and friendly. This is something you could potentially keep up. You book more classes. That's "want to" commitment based on the enjoyment you got out of the experience.
"Have to" commitment reflects a feeling of being trapped, on a plan you wouldn't normally choose. A strict eating plan, a juice cleanse, a 30 day intense boot camp. A class that you know is good for you but you just plain don't like. We buy into these plans because they seem to be selling the motivation we can't find on our own, but studies show commitment levels are low when in a "have to" mindset.
The challenges create a motivation to be consistently in the studio. Ten classes, take home a sweatshirt. 12 classes, a water bottle. I love helping to create the initial why. But what I really love is the challenges continue to reward those who met their why and are still here, at the barre, week after week. Because they "want to."