Why is /dev/null needed to run asynchronous jobs in busybox sh?

I'm curious why this special device is needed to fork the command and run it asynchronously in the minimal Busybox shell. BusyBox v1.30.1 (Debian 1:1.30.1-4) built-in shell (ash) Enter 'help' for a list of built-in commands. /bin/sh: can't access tty; job control turned off / # / # echo Hello && sleep 2s && echo World & /bin/sh: / # can't open '/dev/null': No such file or directory / # / # mknod /dev/null c 1 3 && chmod 666 /dev/null / # echo Hello && sleep 2s && echo World & / # Hello World / #