It's kind of in the word distribution, no? Distros package and ... distribute software.

Larger distros usually do a quite a bit of kernel work as well, and they often include bugfixes or other changes in their kernel that isn't in mainline or stable. Enterprise-grade distributions often backport hardware support from newer kernels into their older kernels. But even distros with close-to-latest kernels like Tumbleweed or Fedora do this to a certain extent. This isn't limited to the kernel and often extends to many other packages.

They also do a lot of (automated) testing, just look at openQA for example. That's a big part of the reason why Tumbleweed (relatively) rarely breaks. If all they did was collect an up-to-date version of every package they want to ship, it'd probably be permanently broken.

Also, saying they "just" update the desktop environment doesn't do it justice. DEs like KDE and GNOME are a lot more than just something that draws application windows on your screen. They come with userspace applications and frameworks. They introduce features like vastly improved HDR support (KDE 6.2, usually along with updates to Wayland etc.).

Some of the rolling (Tumbleweed) or more regular (Fedora) releases also push for more technical changes. Fedora dropped X11 by default on their KDE spin with v40, and will likely drop X11 with their default GNOME distro as well, now that GNOME no longer requires it even when running Wayland. Tumbleweed is actively pushing for great systemd-boot support, and while it's still experimental it's already in a decent state (not ready for prime time yet though).

Then, distros also integrate packages to work together. A good example of this is the built-in enabled-by-default snapshot system of Tumbleweed (you might've figured out that I'm a Tumbleweed user by now): it uses snapper to create btrfs snapshots on every zypper (package manager) system update, and not only can you rollback a running system, you can boot older snapshots directly from the grub2 or systemd-boot bootloader. You can replicate this on pretty much any distro (btrfs support is in the kernel, snapper is made by an openSUSE member but available for other distros etc.), but it's all integrated and ready to go out of the box. You don't have to configure your package manager to automatically create snapshots with snapper, the btrfs subvolume layout is already setup for you in a way that makes sense, you don't have to think about how you want to add these snapshots to your bootloader, etc.

So distros or their authors do a lot and their releases can be exciting in a way, but maybe not all of that excitement is directly user-facing.