Happier horse, he doesn't have urges that he wants to act on, he can live in a herd environment, rather than so many studs who live in solitary confinement.
You don't have to worry all the time about where he is and if any mares are teasing him.
No risk of accidental breedings.
When to geld, I do it ASAP, Stewie was done at 3 1/2 months because he was dropped and ready, I prefer to do at 6 months, but if they haven't dropped then as soon as they are ready they are done.
It doesn't affect their growth, in fact some argue that gelding grow taller, but stallions put on bulk, trouble is that bulk often goes on the neck and front end, not always where you want it.
I can't see any downside to gelding, I am very very unlikely to breed or buy a youngster of breeding quality, so it as never been an issue for me, my question is always WHEN should I geld him rather than SHOULD geld him.
And yes I had people ask about one of my colts, I could of sold him several times as a stud, but was far happier selling him as a gelding, he was a nice colt, a very nice colt, but he never made me go WOW, and if I don't go WOW then he's getting cut