Most often use case in PHP is to detect a country/city of current request's IP address. In common fpm/fastcgi environment database file is re-opened and re-read on each and every request whenMaxMind\Db\Reader instance is created. The idea is to keep database file opened as long as php fpm/fastcgi master process is alive. Simple tests show performance gain at least by an order of magnitude.
As as side note, database should be reloaded whenever its file size or timestamp has changed.