Sometimes the hearer already knows the fact. Reporting it can still convey that the speaker knows it. This is lāzim fāʾidat al-khabar, the ancillary benefit of a report.
For instance, someone tells a person who has seen their examination result, “قَدْ نَجَحْتَ” — You have passed. The hearer gains no new result, but now knows that the speaker also knows it.
The distinction concerns communicative purpose, not two different grammatical species of declarative sentence. One form can provide new information in one setting and signal awareness in another.