png2png.com is for the times when PNG is already the right format. Re-encode a problematic export, optimize it without changing a pixel, reduce its palette for a bigger saving, or resize an image that is carrying more pixels than its destination needs.
The main PNG-to-PNG tool preserves transparency in every mode. There are dedicated tools for file-size limits, resizing, and web-delivery copies in WebP.
Keep PNG when PNG is doing a real job
PNG remains a strong choice for screenshots, line art, interface graphics, logos and images with transparency. Re-encoding can make those files cleaner and smaller without forcing them into a photographic format that introduces edge artifacts.
Use the least destructive change first
Start with lossless re-encoding. If the result is still too large, try palette reduction, then resize only if the image dimensions exceed the real display size. Keep the original until the final copy has been checked on its intended background.