20 Seconds is just long enough to tire most players, but not exhaust them. So when you require a player to focus and put forth maximum effort for 20 seconds, you are asking them to maintain their concentration while slowly becoming more fatigued. They are forced to be consistent and focused for the entire time, and that is when skill development goes into over-drive. Executing at top speed, top concentration – all while becoming increasingly fatigued is a fantastic environment for development.
Additionally, from a score-keeping perspective, 20 sections provides a wide distribution of scores amongst players of varying ages and skill levels.