As mentioned in https://github.com/joshtriplett/bitcoin/commit/82ccb05b07c111223adbbf54e9da7c2a4a185562 , "This quiets down the autotools build, making warnings much more visible."

With --verbose, autoreconf is quite chatty, printing several lines for each tool it runs, including the entirely successful ones.  Without --verbose, it shuts up unless an error occurs, and the only lines printed are warnings or errors.  That nicely highlights the several warnings that do occur in autoreconf, which got lost in the noise with --verbose.