Apart from Final Doom, what are the differences between the standalone Doom 1 & 2 and the ones bundled with the BFG edition?

Is it worth buying the standalone games even if you own the BFG edition?

You can find the differences in the link below:
https://doomwiki.org/wiki/Doom_3:_BFG_Edition#Differences_with_Doom_and_Doom_2

In addition to the differences above, the standalone Doom 2 also comes with the "Master Levels for Doom" mission pack (alongside Final Doom) that is not available on the BFG Edition. Meanwhile, BFG Doom 2 comes with the "No Rest for the Living" Episode that cannot be legally obtained any other way.

I'd say so, especially if you're interested in using mods. There are programs like ZDoom for which you'll need the original .wad-files. I don't think Doom 3 BFG contains these files. (edit: Yeah, disregard that. According to Grargar's post, these .wad files ARE included in the BFG edition)

On the other hand, if you just want to play "vanilla" Doom (with the red cross logos on the medpacks exchanged for pills and Nazi imagery removed from the Doom 2 bonus levels), I guess BFG should suffice.

This information helps a lot, thanks a bunch.