What makes it comfortable?

I think most saddles have a pad over a relatively hard shell. The shell might flex a bit, but basically the saddle action is based on compression which has a variable range of increased resistance. I have a B-17 Special on my single speed flat bar bike and a C-17 on my Roubaix Expert.

My experience with both of them is that I get supportive flex of continuous-value resistance through the range of motion. I get a flow compatible with all the motions that go into riding a bike, especially the nearly invisible one of balancing to keep upright.

My Roubaix came with a Toupe+ (2012 version) and it is pretty comfortable until it starts talking back to my ass at 30 miles. It's compression-based dynamic is more like a dual piston.

Both Brooks saddles show their worth over long distances. After a long ride, my legs are exhausted but my ass could go all night. Wait, that doesn't sound right...

A few weeks ago, for my own amusement, I listed out all the elements of a good saddle fit and the list went to up to 25 considerations before I decided I could be out on a ride instead of doing this. Only 5 of them were about the saddle itself.