IMO, there is no absolute essence because our hobby is so multi-faceted: collecting, designing, constructing, wiring, repairing, operating, making scenery and structures, buying and selling, etc.
Then, there are the emotional/spiritual essences: nostalgia, comraderie, common bond between parent and child and between fellow hobbyists, etc.
This diversity of essences is a very good thing, IMO.
There are multiple essences for me, but if I had to pick one, it would be my by-line: in my little world, I leave this troubled world behind.
Arnold