Boundaries foster creativity. Writing in only seven sentences isn’t so much a rule as it is a definable boundary.
I think of it as a ball park or a football field, the game can only takes place inside the boundaries that we have constructed.
Imagine a game of football, where there was no touch line, where the goals were placed an infinite length apart. The game would never end, it would disappear into oblivion.
Creative constraints force us to define who we are and what we want. Tell a story in only seven sentences, it’s an achievable challenge.