When i export something from GIMP as a gif, there's an option to include a text comment. Never outside of GIMP have i seen this. Googling it just gets questions about people putting gifs in Reddit comments. No other image viewer or editor i've seen makes these visible, and even GIMP only seems to be able to write them, not to read them. So what's up with these? How useful is it to put information in a gif comment?

exiftool can read them. I'd expect other tools that show image metadata would likely be able to read it as well, but I'm not sure about examples off the top of my head.

Testing a few GIFs I have on hand, a few have comments about which tool made the file and that's about it.

https://en.wikipedia.org/wiki/GIF#Metadata

Should be visible if you're looking at the file properties (details vary slightly depending on OS) or you can use something like ExifTool.

It's metadata. Lots of image formats allow you to have auxiliary information attached to the image file like lens info, creation date, location data etc.

People who make use of certain types of metadata will be using software which uses it or displays it, and GIMP kindly provides for this.