I'm not sure if it's still true today, but it used to be that ash was much faster than bash, which was important when a lot of the OS init scripts were written in shell. Nowadays with systemd it's perhaps less important to have a fast POSIX shell interpreter on Linux, but it's likely still useful on BSDs or other OSes that continue to lean on shell scripts in their base system.

Another reason is size. Ash is smaller than bash, so it's useful on embedded systems, and anything using BusyBox (e.g. Alpine Linux) is going to have ash by default.