A packed day can contain plenty of activity without producing much that is useful.
A full calendar gives a day a visible shape. Calls happen, messages get answered, and tasks move around. It is easy to finish feeling occupied without being sure what changed because of all that activity.
Effectiveness is harder to measure in the moment. Some important work looks quiet from the outside and may not produce an immediate result. I do not think every day needs a dramatic output, but it helps to distinguish motion from progress. Otherwise, being busy becomes a convenient answer to the question of whether the day was well spent.