Actually kind of a good question. Most people I meet don't seem to fathom why anyone outside of a veledrome situation would want to ride a bike with no freewheel and only one gear (well 2 if you stand up part of the time). And among those that choose to do so there is such division over what kind of geometry, frame material & gearing make sense & are enjoyable or appropriate to ride in such a manner.
I can only speak for myself and my experience with fixed gears which is so far limited to a few repurposed track bikes on mostly city streets. but even from that narrow category the experience can be radically different depending on a simple thing like a handlebar change. Certainly one of the things that keeps me motivated and interested in riding fixed is how easy and quick it is to switch bars and consequently work muscles from different angles.
I have ridden & enjoyed geared bicycles since early childhood but my first fixed gear bike was an Impulse buy. -KHS Aero track - I first saw her hanging in the shop, around 2005 or 6 (I think this model was from the 90s though) - tig welded, industrial silver metallic grey chromoly with what looked to me like an impossiblly tight wheel base, facilitated by an s bend in the seat tube. It looked pretty crazy to me at first with super deep drop track handlebars and a fixed gear (with which I had no experience) the pedals had clip n strap foot retention... no brakes
it was all new to me but I was strangely attracted. I didn't know anyone that rode any kind of fixed gear bike back then but the exhilaration associated with the danger of the unknown - the fun of trying something new, that I wasn't sure I could do, something that might get me hurt or even killed made me really want to try it somehow, so for $700, which was super cheap for a complete bike even then I took a chance.
And wow it was fantastic! & much more manageable than I'd imagined. I didn't die (although it would have been a lot more sensible to have at least spent the first year learning with a brake) I just pumped the tires to rock hard levels as was de rigueur in those days & learned to ride a whole new way, it felt like the bike was no longer something on which I rode but something connected - an extension of my own body. Sounds bizarre and esoteric until you have experienced it for yourself but at least for me it was great fun. There were new coordination games to play and different cycling strategies required but that was exciting... I found skip stopping to be reliable - easier and more effective than skid stopping most of the time - there were several new skills to practice.
Such an entirely different workout too, as mentioned in previous posts - more upper body muscle recruitment than other forms of cycling I'd done. Going uphill became more anaerobic - downhill effort translated into full flight...--I was hooked. I rode that bike hard until the frame broke Then I spent a little more than twice as much on my next fixed gear - a used Bianchi Pista Concept which was a considerable upgrade and a much more refined experience especially when I went up in tire size to 25c and took some of the air out of my tires...