Why in-browser matters more here than anywhere else
The files people put through these tools are the ones they would least like to hand over: a passport scan being shrunk to fit an application form, a signed contract being split, a medical letter being merged, an unreleased product photograph being compressed for a deck.
Every mainstream alternative uploads that file, processes it on a server and hands back a download. The image and PDF tools here use codecs already in your browser, so the file is read, processed and saved locally. There is no upload wait, and no server-imposed size cap either.
Resize before you compress
The largest single saving is almost always dimensions rather than quality. A photograph straight off a phone is around 4000 pixels wide; displayed in a 600-pixel column, the overwhelming majority of that data is decoded and discarded on every page load.
Resize to roughly the largest size the image will ever be shown at - double it if you are supporting high-density screens - and the file is already dramatically smaller before the quality slider is touched at all.