For when PNG’s own compression ceiling isn’t enough and you still can’t lose any quality. Everything runs in your browser — nothing is uploaded.
PNG’s own compression has a real ceiling, especially on photos — there’s only so much a smarter PNG encoding can do without discarding data. WebP’s lossless mode uses a newer, more efficient method and typically comes out meaningfully smaller than even a well-optimized PNG, for the exact same pixels. The tradeoff: the file is a .webp, not a .png — though since it’s genuinely lossless, it can be converted back to PNG later with zero quality loss either way, if you ever need the .png extension specifically.