Two monitors can be useful if you're paying attention to two windows at once and don't want them to share one screen. Examples include playing a game with Discord open, taking notes for an online class, etc.
As for three, I have no clue.
As soon as I sent that comment I thought expanding your field of view in games would be a nice usage for three monitors, but then I remembered ultrawides exist…
My home setup is an ultrawide, a laptop screen, and a normal 16:9 monitor. To me, it's just handy having a lot of information easily accessible. When playing a game on the UW monitor, I can have Discord and Telegram on one, and GBAtemp open on another. Or any combination of handy stuff. I wouldn't say it's necessary really, but it's just nice to be able to glean information without having to faff about.
I tend to like to play my games in full screen on one monitor and throw and a stream on the other as background noise while still being able to see what happened if I wanted to. bit simpler but still. but ya, the use case of having more screen realestate for windows is kind of the big draw. going back to gaming again, perhaps having the game on one screen and chat for discord on another is also nice.
pushing things to extreme was when I had minecraft open on my left screen and had the world map, my city layout plan, and a live stream open on the right....it's one of the few times I kinda felt like a 3 screen would be neat but at the same time it's rather rare to justify a 3rd screen so probably not worth it...hooray for taking so long to bearly make a point I had to break it into two >.<
But honestly next month will likely be 10 bucks a month.... Dude called his boss and pretty much got a job out of the deal making like 30 bucks an hour.
As for three, I have no clue.