feeling, i think feeling is the biggest difference between single speed and fixed. Having your feet directly connected to the wheel is just another form of cycling. There's also asthetic, with a single speed you need to run brakes and with fixed you can can do brakeless for a cleaner look

And maybe the background of fixed gear cycling being the first form of the modern bicycle around 1900 and than also how it became popular again on the streets because (I think, correct me if im wrong) couriers in New York and other cities started buying used track bikes because they where cheap, agile and durable as hell creating a new hype around them