Same question here. There are some that are rather obvious, like in another thread about spawn humans, it's either 0 or 1, but there are some where I think maybe they give you a performance boost or have different values to make things look different (water -1?).
I have been changing some things, one at a time to see what it does, some where not noticable (usefog), some might just be remants from an earlier stage of the game?, some had a not so good effect on being able to run the game (megatextures).
So if anyone else has tinkered around with these a little bit and has some good or bad experiences it would be very appreciated if you shared.