It’s motivated largely by the idea of convergence, which is combining mobile and desktop together. While an attractive proposition, in reality the execution is flawed at best and awful at worst. A lot of this has to do with the different input methods and screen real estate.
Personally, I prefer the way KDE does it:
Different shells with UIs designed around the different computing paradigms with shared code wherever needed.
While this is also partially the case with GNOME and Phosh, their UIs aren’t really too different from one another. On the one hand, it’s more consistent. On the other hand, one of the paradigms suffers more (desktop in that case).
Unity (8, now Lomiri) was the closest to achieving a balance between desktop and mobile, however it still suffer(ed/s) from many of the same problems but to a lesser degree.
Speaking of consistency, that’s one aspect of Haiku that I like which isn’t implemented well or done too excessively to the point of hindering UX.
Haiku UX is very consistent and uniform, but not to the point wherein it hinders the user too much. The closest thing to this was the old Mac OS, which I guess tracks given that it has roots in BeOS design (from ex-Apple folks).
Haiku can be simple to use by default, but it can also be very powerful when necessary. It tries to make using a computer simple, but not to the point of assuming that the user is a toddler; admittedly that’s a bit hyperbolic, but hopefully it gets the point across. On the Linux side, KDE gets the closest to this aim however much work still needs to be done (it’s happening though); this may also explain why KDE software on Haiku often fits in well with the rest of the system, as both have similar design goals.
Package management is another thing which Haiku does really well, compared to other OSes. It has a good hybrid of both traditional and containerized package managers. It works similarly to (or seems to) OSTree and Flatpak, however still uses shared libs in a way more reminiscent of traditional package management. Also, it’s generally faster than typical containerized package managers in my experiences. Making packages is also relatively simple, in contrast to most other package managers; exceptions to this are AppImage and source-based package managers.
Stability is one of Haiku’s greatest strengths over many other OSes. It’s renowned for being very stable during the alphas and even on nightlies to this day. Meanwhile, other OSes can still get very unstable on so-called stable releases.
Lastly, snapshots are something which Haiku does rather well. Most other systems use separate partitions, filesystems with snapshotting, or both. Haiku needs none of these for its solution, or at least I don’t recall BFS itself having snapshots (please correct if wrong). It should be noted though that Haiku is gradually gaining support for BTRFS, a filesystem that does have snapshotting.