Here should to wait for `@rotate_wait` seconds because some applications write logs to the old file even after log rotation.
For example, nginx opens a log file when a HTTP request comes. If logrotated rotates the log file during the HTTP request, nginx writes logs to the old (rotated) file:

nginx open log file -> logrotated rotates file -> fluentd stops watching old log file -> nginx write logs to the old log file -> those logs are lost.

This happens especially often if users have long-running HTTP requests.