Verbose logs are useful in troubleshooting issues, by default the log levels are at a lower level and this setting increases the amount of information displayed in the logs.

to make verbose logs:
custom settings > Default Standard Objects List > manage > create new > ENABLE_VERBOSE_LOGGING

Note: In conjunction with this setting, you would also have to set the appropriate log level (Errors / Show All) in the display settings (My Calendars > Display Settings > Special Settings > Log events show)