There are really two types of KVMs... classic ones were used by consumers to plug multiple computers into one screen, keyboard, and mouse and flip back and forth between them.

Networked KVMs let you remotely access a computer or server. It's like VNC except by physically plugging into the ports. This is useful because it allows you to have remote access to a server when its OS is offline, tweak bios settings, watch it boot, reinstall the OS, etc.

Most true servers have this functionality and more built in (ipmi, idrac, redfish, etc are terminology for the feature). These plugin ones allow you to add that functionality to any machine. You're not likely to see tons of them in an actual data center, but they are incredibly useful for systems without remote management built in.