There's no "must" about -j, it's optional to spawn nproc [which is a command to return the number of processors] parallel build jobs to compile the extensions. It is simply passed through to make's -j option:

   -j [jobs], --jobs[=jobs]
        Specifies the number of jobs (commands) to run simultaneously.  If there is more than one -j option,
        the  last  one is effective.  If the -j option is given without an argument, make will not limit the
        number of jobs that can run simultaneously.

Really docker-php-ext-install is just a bit of bash handwaving around make: https://github.com/docker-library/php/blob/master/docker-php-ext-install

TLDR: -j$(nproc) make build go fast.