They are quite useful as "VMs at the speed of application containers".

When you use application containers, you tend to create a more complicated setup and lose visibility as to what's going on in the system.

Just like you use VMs on a baremetal server, you use system containers in a VM.

But I find more important to use LXD as a tool for software development and also for Linux desktop use.

If you want to setup nodejs or something similar, it makes sense to put it in a LXD container so that it does not mess up your host. Each step of the workflow (creation, deletion, etc) takes a few seconds or less.

You can also setup LXD containers to run GUI applications. It makes big sense if you want to run games (like Steam) so that your desktop does not get polluted with i386 packages. You can also use for development tools, such as those from JetBrains, Android Studio and more.

They may not be mainstream but I use them as isolated VM-like application environments, where everything “just works” without having to learn/apply a whole lot of new tools or workflows.

I’m sure there must be others who see the benefit of this approach too?

They're certainly very useful for CI systems.