If you didn't know about "Need To Know," you now know.

A scene in the movie "Zero Dark Thirty," captures "Need To Know," perfectly. The central character, Maya Harris, is having lunch in the CIA cafeteria when the Agency Director sits at her table. They make small talk until he asks her a detail about the Top Secret program the agency had that recruited her, right out of high school. Since their conversation had not established his need to know this information, she tells him that she is not allowed to tell him. Had she presumed that "he was the Top Boss," and answered his question without a challenge, she would have lost her job, and probably have faced charges.

What "need to know" did Biden have for the classified documents that were found in 3 different places under his responsibility?

Originally, he had the need to know the information as Senator and Vice President. After that, he or the staff member responsible for such things forgot to return them and they were kept in Biden's papers in his personal files. Biden's first batch of documents were discovered by is staff while moving documents from one location to another, and a second batch were stored in the garage of his home in Delaware. Like former Veep Pence, he has returned classified documents to the appropriate officials. There are no records of either Biden or Pence refusing to return documents, when requested. In both cases, they appear to have lost track of them. Pence has explained his lapse to the satisfaction of investigators and that case is closed. A Special Prosecutor was assigned to investigate the Biden papers. The layers that adds to procedures is likely to delay a resolution and report in this case.

who had access to these documents and did they have the appropiate clearances to view them?

I guess we will know when the Special Prosecutor files their report.