Auto-post joins the last two stages of a document's life. With auto-post on — it is, by default — the moment a document is approved, it posts: the entries are written, stock moves, and the document arrives at posted in a single act. With auto-post off, an approved document rests in approved until someone with the post permission posts it deliberately.
Because approval is the authorisation, the approver does not need the post permission for auto-post to work — the system posts on the strength of the approval. Two consequences follow. First, the approver is the last human gate: with auto-post on, approve carefully means exactly what it says. Second, a failed posting cannot leave a document half-approved: if posting is refused (a closed period, an insufficient bank balance), the document remains a draft and the approver sees the reason, so the work returns to the person who can fix it.
The benefits, in companies that use it — most do — are practical: documents never queue overnight between approval and the ledger; stock on screen matches stock in the yard; statements and cockpits read current figures rather than yesterday's intentions. Keep auto-post off where your controls genuinely require a second pair of eyes between the decision and the books — some finance teams hold postings for a daily review — and on everywhere the approval itself is the control you designed.