Same as any update for any other OS - Fix bugs, patch security, add features.

But there's more to it.

If you come from a non-linux background, you may not be aware of the division in responsibility between what is your distro's concern, and what is not.

The thing that people think of as the "operating system" in the sense of Windows or Mac is likely really just the desktop environment - the stuff you can see and click on; your taskbar, control panel, file explorer, etc. In Linux the desktop environments such as Gnome, Plasma, xfce and many others are built by separate teams and used in many different distros, and so changes and improvements in those aspects won't necessarily be part of the distro's release notes even if improvements are happening.

This modular nature of Linux is likely a big part of why it doesn't seem like much is announced, even on major updates, because it's not your distro specifically which is responsible for that.

As an example, Pop_OS! are working on a new desktop environment called Cosmic, and some of the release notes for that may sound more like what you'd expect.