As a developer, I find the great bang for buck is two monitors, with minor benefits for each additional monitor. Examples of use for extra monitors:

• code on main monitor (or more if I have multiple projects open)
• app I'm working on
• documentation
• Stack Overflow/Search/AI chat
• Team chat/Slack
• Email
• infrastructure/performance monitors, logs
• Mandatory reboot notice from the IT department shoved in a corner until I can reboot without losing work

Of course if you have a large monitor you can tile windows instead of having N screens. Virtual desktop also exist.

Common other uses for multiple monitors:

Usually your main monitor is in landscape mode, so a monitor in portrait mode can be used to view material that works better in portrait. I use one to read manga. Yes, you can also do two pages side by side in landscape, but I prefer it larger, especially with manga which often has tiny details that are interesting.

If you are using full screen mode on your main monitor, then you can have other information on your other monitors. Lots of people play games and have their voice chat apps on other monitors so they don't need to overlay them on the game. Or you can have notes open or something.

Lots of people like things in the background like tv shows or livestreams or podcasts. I find that distracting, so I don't do it, but a lot of people do.

Sometimes, depending on your profession or hobbies, it's good to have certain notes always visible, when they would normally get hidden behind other windows on your primary monitor.

A few desklets you say... There's always more to be displayed! The area between the left and right row of applets is used for Dolphin (explorer) and fsearch in active use.

1st monitor: Browser or Game depending on circumstances.

2nd monitor: Security camera feed

3rd monitor: 3D printer controls/status can be swapped for discord whatever I switch to from discord.

4th monitor: Home assistant control panel