The largest section, because "I have a picture and it is wrong somehow" covers a lot of separate problems. It is too big, the wrong format, the wrong shape, it has something in it you want to draw attention to, or it has data in it you did not know was there.
All four tools here use your browser’s own image pipeline to decode and re-encode, which has one consequence worth knowing: the size limit is your device’s memory rather than an upload cap, and the exact output differs very slightly between browsers for the same settings.
The metadata tool is the odd one out and the one most worth reading about. It does not decode the picture at all — it edits the container, dropping the blocks that hold EXIF, GPS and similar data while copying the compressed image data through untouched.
What people use these for
Shrinking a 7 MB phone photograph to something a web page can serve.
Converting a PNG screenshot to JPEG, or the other way round for the right reasons.
Checking whether a photo you are about to post publicly contains GPS coordinates.
Removing the location and device data from pictures before a marketplace listing.
Cropping to the shape a platform expects, with a preview of where its interface will sit.
Annotating a screenshot to show somebody what you mean.
Privacy in this section
Nothing here is uploaded. That matters most for the metadata tool, whose entire purpose is showing you the GPS coordinates and device identifiers inside your own photographs — sending those to a server to have them removed would defeat the point.