[From memory, so take with a grain of salt] a) tooling and familiarity b) it's leveraged for modularity, composition and access control. Normal unix basically

[From memory, so take with a grain of salt] a) tooling and familiarity b) it's leveraged for modularity, composition and access control. Normal unix basically has no modularity and little composition beyond piping to stdin (which is of limited applicability). And of course no useful access control mechanism to speak of. The last 5 decades mostly just added layers of shitty hacks that don't work and that no one understands anymore (by contrast e.g. the original unix ownership and permissions model was misdesigned but at least possible to grasp). Plan9 does substantially better in both regards via clever use of a proper union file system (which incidentally makes another horrible hack superfluous: symlinks). If you want your shell, or some other executable, to find some executable, you mount it into its /bin -- there is no $PATH. If you want some process or user to have access to a resource (file, device, server in the network, ...) you mount it into their filesystem. This gets rid of a lot of problems and special purpose solutions.