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Because progress is reported at standup as narrative. Everyone says things are fine until the Thursday of the second week, when it becomes clear they are not.
A burndown chart plots remaining estimated work against remaining days in the sprint.
The line is remaining estimate on open tasks, recalculated as work closes. Scope added mid-sprint shows as the line going up, which is the honest picture.
Four days in, the line has not moved. Everything is in progress and nothing is done — usually a sign work was started in parallel rather than finished, and it is early enough to change.
It will not tell you why. A flat line is a symptom — blocked work, oversized tasks or optimistic estimates all look identical on the chart.
It reads the sprint's tasks and their estimates; nothing is entered for it.
Yes, as an increase, rather than being hidden.
Yes, by estimate or by count, per team preference.
Yes, across sprints toward a milestone.
Fourteen days, every module, no card. Or half an hour with someone who will run it on your own records and tell you where it does not help.