Why convert a PNG to SVG at all?
A PNG stores a grid of pixels. Scale it up and the edges get soft; scale it down and fine details disappear. An SVG stores shapes as curves, so the same file looks sharp on a phone icon and on a billboard.
SVG Harbor traces each colour of your PNG separately into smooth Bézier curves, keeping sharp corners sharp, then stacks the layers so they overlap slightly instead of meeting edge to edge. That removes the white hairlines you often see in traced files.
Does it work for photos?
Partly. A photo has thousands of colours and soft gradients, which don’t map well to flat shapes. With the Photo poster preset you get a stylised, poster-like vector with a handful of colours. For logos and icons you get a faithful copy.