Some (but not all) of the reasons why the kernel is updated are:
- new features
- new drivers, more or better hardware support
- security fixes
- documentation updates/fixes
- improvements, better ways to do an old thing
The kernel is being updated and improved every single day. If you want to see what moves inside the linux kernel, subscribe to the kernel mailing list. However, keep in mind that it's a very
heavy traffic list. That can mean +300 posts in a single day. You can also check the changelogs.