As AI improved it's ability to generate varied content types, a rendered preview allows user to instantly see not only the initial result but any iterations made as well.
Likely for more technical users, this pattern is most commonly used for coded results which require a rendering to actually see & review the results accuracy.
A general understanding is needed that this is a temporary preview & that some differences may apply when applied eslewhere.
Not all file types or result require a preview. Defining what presents in the preview pane is important. A preview is highly effective when relevant but adds friction when it's not required.
Another consideration is result complexity or length. Having AI generate a single JS function may not require a preview. Perhaps proviging inline options for the preview instead adds user autonomy.