JPG stores photographs with a quality setting. Fine detail and flat color can show blocks if the quality is low. A second save at a lower quality throws more data away.
PNG is lossless. Logos, screenshots and images that need a transparent background usually belong in PNG or WebP. A PNG of a photograph is often larger than a JPG of the same picture.
Re-encoding a JPG to PNG does not recover detail that an earlier JPG save removed. It only wraps the current pixels in a lossless container.