You can make decisions without having to think about whether you'll be able to do the same for the next child when they reach that stage - whether that's riding/music/ballet lessons, special outings, expensive school trips, visiting university open days for four consecutive weekends. You also avoid the problem of two children wanting to do different after-school activities at the same time and having to decide who wins.
You can do more adult things with one child than two, because when they're bored of it you can go, without the problem of one child being interested and the other bored. You can go places/eat out without risk of bickering, because there's nobody to bicker with. You can do things with them where they need one-to-one help. You can watch family movies easily - no telling the 13 year old it will have to be a PG, or scaring the younger one with something they're not quite ready for.
Reciprocal childcare is much easier - if you normally work school hours but have a later meeting one day, it's relatively easy to organise a playdate for one child, harder to farm out two. (Even easier in my case as DD had friends who were also only children with mums who occasionally needed the same in return.)
Once they start going to playdates and parties without you, you get a couple of hours off.
No bickering in the car, plenty of space for holiday luggage even with a small car.
When they get old enough to decide where they want to meet up, a house where there aren't any siblings around can be an advantage.
I was one of four, and didn't have a problem with that. DH and DD are both only children, and both prefer that - they are both quite happy in their own company. I think it might have been better for DD to have had a sibling during lockdown, but she says it wasn't a problem.