What is difference between Fallout from store and Fallout Classic from my library? Same question for Fallout 2 and Tactics?

for the most part its just that the classic versions have extra goodie items

There is no actual Classic in your library. Only if you download mac pkg archive (and extract win version from it).
There is summer 2015 update to Classic that make it equal to non-Classic by bethesda. with bethesda laucnher. There is differences now, cause non-Classic got more updated launcher and include optional Hi-Res community mod. Classic do not have that feature.
Every version is based on EU version (censored).

A couple other differences:
* I believe Fallout Classic has more extras than Fallout.
* Buying Fallout will get you TES: Arena and Daggerfall. (Note that this is not a good reason to buy Fallout if you already have Fallout Classic; Battlespire is cheaper and also comes with those games, and if you want something like Daggerfall (albeit with prettier graphics and a smaller, but more interesting, world), you could just get Morrowind which comes with those two games as well.)

I don't really understand what's going on here... GOG delisted the original games, then relisting them as separate titles, minus the original goodies? What is the point of this other than annoying consumers by hiding the "already in library" notification when browsing the storefront?

This might have something to do with the publisher's decisions and agreements. Both Fallout games were completely delisted for some time. After a break they reappeared as a new agreement with Bethesda was made.

There was also a mac version of each game as part of the classic package - unfortunately they didn't work on newer macOS. Good news i guess is that they were made with wineskin - which you can do yourself easily enough.

It contains the bethesda launcher they bundled with the "fallout anthology" dvd release, and high res patch. It's the UK 1.2 version, so it's censored, and only comes with the manual pdf. I did some googling and found some links:
TeamX updates --> http://www.teamx.ru/site_arc/patches/index.html
High res patch --> https://www.nma-fallout.com/resources/fallout-1-high-res-patch.52/
Other goods --> https://pixeldrain.com/u/K2LgkeRX
Patch to fix censorship --> https://www.moddb.com/games/fallout/downloads/fallout-12-censorship-fix-gogsteam

Edit: after checking over the game files, it looks like some of TeamX's fixes have been included with the current gog release. Not sure if that was the case for the 'classic' version, but these fixes are in the link above.

classic versions have had fixes pre removal.
if you were lucky the games were free

Ok then.

I have all the 3 games in classic version (including all the goodies) so I don't think it's warranted for me to buy them again.

P.S. Currently playing through Wasteland 2 and the post-apocalyptic itch needs more scratching. So probably I'll move to the Fallout series again after I'm done.

Odd but the versions I have don't seem to be censored at least not like the original UK release. Fallout 2 still has children and the Frank murders a whole family cut scene is still there. Tactics still has the gore, language, hookers & drugs. I got mine a long time ago because I was in the Beta for Gog.