A lot of cyclists enjoy cycling above Zone 3. One time I went on a ride with just one person and he was max zone 3 while I was flipping between zone 4 & 5. I’ve gone cycling with people slower than me and it’s literally so boring. You either go at your natural pace and wait at a lot of stoplights for them to catch up or you pedal way less.
Speed also changes where you can go. Where I’m at, Cyclists will often meet up at 5am or 5:30 am and go on a 60km ride together before going home and getting ready for work. 25km/h and 30km/hr results in a difference of almost half an hour, which many people can’t afford to wait.
I’ve gone on century rides with a friend where we did 60km ride to the base of the mountain, then 1km elevation over 10km, then back down, and then back home totaling around 150km.
In summary, speed literally changes how long the ride takes by hours and what routes are available. You love the feeling of the wind and the beautiful views, and the longer you go the more interesting routes you can go on. Most probably have a time constraint, so higher speed = more interesting routes and more fun.
There’s also groups of people (mostly 50 and above) who are past their prime but still in the community. It’s nice to join them on bike rides during your rest days where you’re doing active recovery.
Cycling, and other endurance sports, are great in that you can go on group rides with people at different skill levels by pairing your fast days with their slow days and vice versa until you finally are able to catch up to them on their fast days.