What makes a PNG actually transparent
Transparency in a PNG comes from the alpha channel, a fourth layer alongside red, green, and blue that records how see-through each pixel is. A JPG has no alpha channel at all, which is why converting a JPG to PNG never produces transparency — the background has to be removed first, then saved as PNG-24 to preserve the soft, partially transparent edges around hair, fur, and fine detail.
This tool outputs PNG-24 by default, which is the format that keeps those soft edges intact. A flat PNG-8 export would work for simple logos with hard edges, but it turns photographic edges into a jagged outline, so PNG-24 is the safer default for anything that came from a camera.