Improvers level in Line dancing appeared as an in-between level for dancers moving from Beginners to Intermediates. Some called this level Easy Intermediate but today it is a separate level.
Why do we need Improvers in Line dancing?
I believe that Improvers came into being because Intermediate dances got more complex and faster. The mix of these two things means it is difficult for a beginner to make the leap from Beginner straight to Intermediate.