> what is "atomic and transactional Linux"?

Linux distros that are updated with full system snapshots instead of package by package, similar to Android. The key difference is most of / is mounted read-only[0] and is only changed by distribution provided updates so you and the distro team always know exactly what's running.

> What are the advantages to the alternatives?

Greater control and stability since its essentially always running in a supported configuration. Easy roll-backs to a previous update if something goes wrong. You always know exactly what your system is running if you want to keep it in sync across machines (more useful in a server setting).

> What other projects are similar

Kalpa is a "sibling" project to AeonOS, which is atomic OpenSUSE but with Gnome (and other changes, which I'll get to). There's also the Fedora Atomic line of Fedora Kinoite and Silverblue (KDE and Gnome respectively), U-Blue, Bazzite, SteamOS, and more. I think most major distro lines have an Atomic variant at this point.

> What is the motivation for this project in particular?

For Kalpa specifically, it's to offer a KDE alternative to AeonOS. Originally there was just AeonOS, which was OpenSUSE MicroOS (an atomic version of OpenSUSE Tumbleweed) with GNOME installed. Aeon has diverged greatly from MicroOS though and I think it no longer uses it as an upstream. AeonOS also refused to support KDE[1], so Kalpa was created. Kalpa still uses MicroOS as its upstream and I'm not sure if there's any plans to change that.

> Most importantly, why should I want to use it?

I use it on my personal laptop because it lets me have all the benefits of a rolling distro (up to date packages) without the stability concerns. Updates apply automatically in the background and I know when I reboot I'll always have a working system available to me.

[0] /etc is mounted as an overlay FS so you can still make changes to it. /var, /usr/local, and /srv are also still user-writable. I think /mnt is too but I forget off hand.

[1] Aeon is generally anti-customization and does its best to only offer one way of doing things. This is to prevent configuration drift and reduce the maintenance burden per snapshot. GNOME also has a more regular release cadence, which makes it much easier to integrate than KDE (or so I've been told..)