Joomla stores its session metadata in the #__session database table — as long as the “Track Session Metadata” option is enabled in your site's Global Configuration.
Joomla does this for all user sessions, even Guest (not logged in) sessions. This also means that each and every hit by a search engine counts as a different session since search engines do not store cookies, therefore do not let Joomla keep track of a single session when they are indexing your site.
On sites with a lot of traffic, even if it's traffic from Guest users, this can create a lot of “dead” session metadata table records, i.e. a lot of records on that table which do not correspond to active sessions. This can inflate the size of the session metadata database table, making your site slower.
This task type can locate these records and remove them, keeping the size of that table in check, ensuring that your site's performance will not degrade over time.
We recommend running this task every twice as much as your session lifetime as defined in the Joomla Global Configuration but not less than once per day. For example, if your session lifetime is 60 minutes (one hour), run this task once every two hours. If your session lifetime is 1440 minutes (one day), run it once a day — not once every two days.