They’re just plain good fun. For me it takes me back to being a kid and just blatting about on a bike for fun without really caring about the bike itself.
Allow you to focus on enjoying the ride without having to bother about shifting etc. Will improve your bike handling and fitness no end. Sure there will be stuff you can’t ride up, in which case just carry / push it. You’ll probably still be quicker than someone fiddling away in the granny gear.
Clearing a big climb or a difficult tech section on a singlespeed will put a huge grin on your face, especially if you beat your mates on gearies. Enjoy. 🙂