When and why would one do that to solve what kind of problems? (Honest question)
One situation would be if you need some driver functionality that was just implemented and hasn't trickled down to distro packages yet.
Yeah. In case of Debian freeze, if Mesa won't be updated in experimental, it can be a good way to keep it up to date for games. Also, there can be cases when you want to test some proposed patch, which isn't even in the official git source yet.
Brilliant job! Very useful for Radeon and Intel video card users.
Thanks! As Mesa is getting better and better, I expect a growth of AMD + mesa usage. I just hope that developers will now pay attention and actually test their games with radeonsi / radv, instead of saying "we support Nvidia only". Intel is somewhat less of an issue, since it's not going to help very demanding games anyway, but if developers could test Intel as well - all the better. Lower / medium end games for sure should do it on Intel too.
As GOG maintainers, you can encourage developers to do it and remind those who forgot about it, that Mesa is now something that they can't be ignoring.
There is now a community list for games broken on Mesa here: https://www.gamingonlinux.com/wiki/Games_broken_on_Mesa