Work Plans are scheduled work logs, in other words, they serve as a blueprint for tasks and keep track of remaining time for a user/issue.
The main goal of Plans is to precisely track Spent time vs. Planned time (accuracy) by users. When a user logs work with a plan, the spent time is added to the work plan.
Accuracy measures Spent time vs. Planned time:
0% -> XX planned, nothing spent
100% -> spent = planned
> 100% -> spent > planned. Underestimation.