Diamond Editions extras has this at 142mb, what is it exactly?

It will unlock the three premium modules that comes with Diamond but are likely encrypted and unusable until you run this.
Kingmaker
ShadowGuard
Witch’s Wake

You may need your CD key to do this. Can get by logging into the GoG website going to games, NWN Diamond, "More" cd keys. If not there would have to contact GoG support.

cheers. I'll backup the cd key along with the game offline installers.
Any other games that I should maybe get cd keys for whilst I backup my offline installers?

I do not know them all. There are a number of posts on the main forums about it though.

Quake 3. One of the Icewind Dale games. NWN and NWN2 of course. Spore. Uh... have one or two more in my library but can't remember.
It is a fair assumption most DOS based old old games probably don't have a serial key.
Anything from DOS in mid/late 90's where CD drives were becoming a thing and into 2010's would be fair game.
But there are some, like Bethesda games (Oblivion/Fallout/Morrowind) that no longer require keys.

Just have to go thru your library one at a time and check! Worse things you could be doing.