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.