How an existing post ad works
When you build an ad in Ads Manager you normally upload a new image or video and type fresh copy. Choosing an existing post instead means pointing the ad at something your Page or Instagram account has already published. The ad then serves that post, exactly as published, to whichever audience the ad set targets.
The reason to do it is that the post keeps its identity. Reactions, comments and shares stay attached to the post rather than to the individual ad, so a piece of creative that has already collected a conversation carries that conversation with it. Build the same picture and caption as a new ad and you start from silence.
Why existing posts matter
A stranger scrolling past an unfamiliar business has almost nothing to judge you on. A comment thread with real questions and real answers is one of the few signals available in the frame itself, which is why social proof is the main argument for this method. It costs nothing to keep and it compounds while the ad runs.
It also joins up your organic and paid activity. Anyone who reacts to or comments on the promoted post can be gathered into an engagement audience later, and the discussion sitting under the post is honest feedback about your offer that no report will give you.
Where existing posts go wrong
The engagement is not automatically an asset. Public comments cut both ways, and an unanswered complaint or a row about pricing is shown to every new person the ad reaches. If you promote a post you must moderate it daily, in the languages your customers actually write in.
The second problem is that you are advertising something built for the feed. A post composed as a square photograph will be cropped or padded when it appears in a vertical placement, and the copy cannot be tailored to each audience without breaking the very thing you were trying to keep. There is also an operational risk: the ad depends on the post continuing to exist, so an over-eager tidy-up of the Page can take a live campaign down with it.
How to act on it
Use it when one message suits everyone you are targeting and the value of accumulated engagement is worth more than tailored copy — a launch, an event, a piece of news. Publish the post, check how it renders in each placement you intend to buy, then attach it to the ad and leave the post alone.
Where you need several messages for several audiences, build separate ads instead and accept the cold start. And if you want the engagement pooled across a whole campaign rather than lost each time you duplicate an ad set, the technique to read next is post ID advertising, which reuses one post identifier everywhere. Either way the underlying work is still the creative, which is where Facebook advertising is usually won or lost.