The history is that Aurelien Pierre was leading some big changes in Darktable around the time the filmic module was introduced and the bigger switch from rethinking the internal architecture and processing pipeline to be happening in the right color spaces before things get crammed down into the RGB display color space. There's a lot of subtlety to that. But it boils down to a lot less rounding errors and loss of information for basic edits related to color correction, tone mapping, etc.

Aurelien Pierre was the main developer of filmic and several related modules and a key driver of that whole transition. A few versions ago there were some other developers that forced through a bunch of changes that in his view took the product in a poor direction in terms of UX. He argued against that and people ignored him and he basically decided to do it properly with Ansel and strip away all the stuff he never liked as well as beat some sense into especially the lightroom part of Darktable which is where you organize your photos. IMHO he did a great job on that from what I've seen so far.

Since the fork, a lot of stuff happened of course and there are now several competing alternatives to filmic in Darktable (Sigmoid and Agx). They each have their pros and cons and the differences can be very subtle; especially if you are new to the topic. There's a lot of complexity here. Which is why UX matters. And a point of criticism with especially Filmic is that it is a very complex module. But I'm still on Darktable and Ansel is actually not compatible with the newer Darktable versions.