The honest reason to do this
Converting JPG to PNG does not restore any quality lost when the JPG was made; nothing can. What it does is stop the bleeding: if you’re about to edit, crop or re-save a photo repeatedly, working in PNG means no further loss with every step.
It’s also a reasonable choice for small collections you want frozen exactly as they are. Just know the files get bigger, because PNG refuses to throw anything away. For big libraries, staying in JPG saves far more space.