Your accounting software shows a healthy bank balance. You glance at it before approving an inventory order and move on.
Nobody has compared it to the actual bank statement for four months.
The number on the screen is a guess with decimal places.
Somewhere in those four months, a transfer between accounts was recorded twice. A card refund was never entered. A payment that bounced is still showing as received.
None of it is dramatic. All of it moves the balance. And every decision made from that screen since has been made against a figure that does not exist.
Reconciliation is the step that turns a guess back into a fact.
It is also the step most often skipped, because it is slow and nobody sees it being done. That is exactly why it is the first thing the diagnostic checks.