An estimate of a task’s duration can be derived from historical data, expert knowledge and even guessing. Most estimates make allowances for “stuff” that often occur. Things going wrong, other things to work on, distractions and …? Historical data may be available for a task that has been done many times.
The estimate is put in project management software which gives it a start and finish date. The estimate becomes a due date, a deadline. People work to meet deadlines. Deadlines make it difficult to get done early.
I used to work to a due date report for my projects. The project durations were based on standards set by the company. Whichever of my projects was nearing its due date got my immediate attention. I almost always finished them on the due date. It was amazing how those standards had such accurate durations.
I suggest that the duration estimate is a number that is useful for planning the project. During project execution, it becomes a deadline.