The calendar runs to a race, not to a month. A twelve-week block ending on a specific Sunday means enrolments, payments and progressions all follow that date, and a membership billed monthly cuts across it awkwardly.
People move between groups constantly. Somebody improving belongs with a faster pace and somebody returning from injury with a slower one, and a fixed allocation made in January stops describing reality by March.
An injury rewrites a plan. Recording that somebody is running reduced volume, and until when, is what stops a coach prescribing intervals to a person who should be walking.
Knowing who went out matters. A group running on unlit roads in winter needs a register of who started and who returned, which is a safety measure rather than an administrative one.