to visualize time gaps between things
We exist in different timelines at the same time.
Calendars usually have rigid timeframes: week, month, year etc. But important things often span arbitrary timeframes.
Here, timelines are first-class constructs, so you can display task trees, timelines of different resolutions together.
Besides from keeping track of deadlines, timelines rewire us to think on longer time horizons.
to mark it as complete
On most calendars, tracking small habits take up too much space, creating visual clutter.
In actions.life, you can display habits compactly with icons (or draw your own).
and write about anything you've done
Anything can have writing and photos attached. Instead of keeping a separate journal, it's often easier to journal directly on the task itself. The calendar becomes a contextual life log for not just events, but memories.