When some softwares(including stunnel) receive SIGUSR1, they reopen a log file without re-reading configuration files or restarting. Since httpd uses SIGUSR1 for graceful restart, it could use SIGUSR2 purely for reopening log files.
Why do people need this signal? A graceful restart doesn't kill active children processes, and those processes still write on an old log file. logrotate can be configured to compress old log files, in which case httpd would try to write onto a compressed log file. Since httpd can't log to a file, it would show abnormal behaviors or even abort downloads from httpd. So SIGUSR2 can come in handy.
