For (almost all) photos, JPGs are smaller and therefore load quicker. Its compression is much less noticeable in photographs, and therefore the size savings are usually worth it.
For logos, text, geometric imagery, etc, you generally need the image to remain relatively undistorted, so PNG is the way to go.
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What is the purpose of using JPGs instead of PNGs.
For (almost all) photos, JPGs are smaller and therefore load quicker. Its compression is much less noticeable in photographs, and therefore the size savings are usually worth it.
For logos, text, geometric imagery, etc, you generally need the image to remain relatively undistorted, so PNG is the way to go.