Saving never overwrites
Content records are versioned. When you save, you do not replace what was there — you add a new revision alongside it. The old one stays exactly as it was.
That single fact changes how it is safe to work. You can rewrite a page substantially without risk, because the previous wording is still there and can be made current again in a couple of clicks.
The active revision
Exactly one revision at a time is active — the one that renders. Saving normally makes your new revision active, which is why edits appear immediately on a published page.
Making an older revision active again is how you undo. It does not delete anything: it points the record back at the earlier version, and your newer one remains in the history.
Comparing versions
The revision history lets you compare two revisions and see what changed. This answers the question that otherwise causes arguments — "who changed this, and when?" — without anyone having to remember.
Where content actually lives
Content belongs to the revision, not to the record. The record holds the things that identify it: slug, position, parent, publish state. The revision holds the name, the layout and every area's value.
This explains a behaviour that otherwise looks odd: changing a page's slug does not create a new revision, because the slug is not part of the revision.
Housekeeping
Revision history grows indefinitely on frequently edited records, and sites are often configured to purge revisions older than the active one after a period of inactivity. If you are relying on a specific old revision, do not assume it will be there in a year — copy the content somewhere durable.