For some tools, it could be handy to show output before completion.
Some fixers could ask if some corrections should be done. (phpcs -a is similar, it waits for manual fixing and rechecks.) Some legacy code could be used. Showing progress of a tool could be another use case.

I plan to call my current pre-commit-shell-scipt (a bash script) with pre-commit (this project). Later I would split up the script and use implementation of #871 ;-)

verbose: full
# or 
verbose: linewise
# or
verbose: buffer 2 # write when no new output after 2 sec