If they don't need to upload it why are they using a web browser at all? There are literally hundreds of programs that integrate tools to compress images.
Because you're in a place where you don't have the right to install new software.
Because it's quicker.
Because it's OS independent.
Because you don't need to update it.
The website worked and I moved on with my life. I'd probably even still be using Youtube to .mp3 websites had I not happened to have experience with youtube-dl.
There are GUI tools for image compression but mostly they're wrappers around the libraries that drive the apps I've listed, but a few versions behind. They're not great.
Using a browser based app or service is ideal for compressing a handful of images.
Also running any script in your browser is downloading, and running anonymous code giving it a playground to do whatever it wants. I'd trust the guys maintaining imagemagick or ffmpeg in my little corner of the web over the security team for firefox. Smaller attack surface y'know?
Most people don't have imagemagick installed or know how to use the commandline, and even people who do might want to quickly resize an image on a friend's computer, or something.