I'm on Linux, and while there are some alternatives, Gimp is pretty much the defacto choice for raster editing. I used to do Photoshop on Windows up until version 4, but switched to Linux and Gimp at that point and haven't looked back.
At this point any feature-rich photo/raster editing software will have all of the functionality 90% of users need, so I don't think you'll get a response with some killer feature that's exclusive to gimp. So for me it's a matter of...
It's a feature-rich editor, with a standard plug-in format, and a ton of file import/export options.
It adheres to file standards quite well.
It has a large user-base, with a ton of tutorials, community support, and add-ons available.
It's available on many different OS platforms.
It's Free Software.
It's free software.
It's local software that doesn't require a subscription.