PNG to JPG is almost always about size. A flat UI screenshot or exported graphic that weighs several megabytes as a 24-bit PNG often drops to a few hundred kilobytes as a JPG — the difference between a page that loads instantly and one that stalls on mobile. The trade-off is transparency: any see-through pixels are flattened onto a solid background, white by default.
Reach for this when the image is headed somewhere that rewards small files — an email attachment, a marketplace listing, a CMS with an upload cap, or a blog post you want to score well on Core Web Vitals. Keep the original PNG if the artwork has crisp edges or text you may need to re-export later.