More detail
The system is built so you do not have to file by hand, but an outage can leave invoices queued. The closing check is simply making sure that queue has cleared.
If anything is still pending, it is usually because the network is down, and it will file on reconnection. Knowing it is queued, not lost, is the reassurance the close gives you.
Related questions
What if an invoice did not file?
Do I have to file manually at close?
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