The compelling reason is simply:
You learn the ideas that programs like Gimp and Photoshop allow you to do to manipulate files; you don't
"learn a program". Hours and days are nothing compared to the fact that you never really "learn" it all.

I have Gimp installed (on an external drive) as well as RT and a couple of others. I also have FastStone, though
I use this a lot to look at raw files on the card before transferring them to PC. Call them all a legacy of trying
things out that members use on AP. (So if someone has a problem in Gimp, say, I can go and try it out.) But I do not have
the raw software like DPP and V/CNX as I don't use Canon and Nikon (or most other) raw files.

My raw files > Σ SPP > CS2 Pshop.