Can't speak for the project members or main users, but as an alternative OS nerd who actually used BeOS R5 on a 300 MHz Pentium II in-period I see Haiku as having two different "purposes" depending on version.

The x86-32 version (and hypothetically the never-complete PowerPC version), as I see it, exists (or would exist) for binary compatibility with legacy BeOS systems. The AMD64 version on the other hand is a hobby OS demonstrating a path not taken where personal computer operating systems remained separate from server operating systems.

Also, like others, these days I can do basically everything I need to do on a computer other than gaming as long as I have a browser that supports the modern web and a SSH client so Haiku is absolutely fully usable on the right hardware.

IMO For some it is practically usable with an ever-growing repository of new and familiar packages. HaikuPorts has over 4500 packages.

For the longest time there was not a modern browser that could run, but now there are multiple chromium-based and firefox-based options.

i can't speak for the project's maintainers and their motivation, but it is workable as a daily use OS if your hardware is supported and you are willing to use the still beta firefox port.

It appears to be usable for daily use for some people, in that enough of a web browser works that you could mostly get by. It would be hard to say it is really practical, nor that it has a convincing path to being practical in the way that say ReactOS does.

Most people working on kernel/osdev do for the sake of recreating it :)