Doubt about Session - sess_time_to_update

[eluser]soprabaixar1[/eluser]
What effect does the config "sess_time_to_update" (config / config.php)?

I read in the documentation and did some testing, but I could not understand the advantages and disadvantages of using this configuration.
Bearing in mind that whenever you add or change or delete the session the session already updates the database

I know that it changes the session_id in the database, but with what purpose?

[eluser]WanWizard[/eluser]
This is a good start: http://segfaultlabs.com/files/pdf/php-se...curity.pdf