There can be several reasons for that. Say your project doesn't only require gradle but other tools to be run too. Or say there are some post-scripts or pre-process-scripts that need to be run. They can be done easily like:

  build pre:
     ! echo 'pre script'

  build proj:
     ! gradle hello

Plus, you might want to run some python scripts or run some other shell commands during or after you have built your gradle project.