The primary purpose for documenting lessons learned during project closure is to:

The primary purpose of lessons learned is to create a knowledge base that can be referenced for future projects, allowing subsequent teams to learn from past successes and failures, thereby improving efficiency and outcomes.

While lessons learned are part of the closeout report, completing the report is a procedural step, not the primary benefit or purpose of the lessons learned activity itself.

Identifying post-project needs for functional teams is typically part of operational handover and might be informed by lessons learned, but it's not the primary, overarching purpose of documenting lessons learned.

While project deficiencies might be identified, the purpose of lessons learned is not solely to report negative aspects to the sponsor, but rather to extract broader organizational learning.