It is to help avoid notifying for group join when using the https://buddydev.com/buddypress/introducing-bp-group-activities-notifier-plugin-for-buddypress/

buddypress-stop-group-join-notifications.php

//Do not notify for join group activity in BP Local group Activity notifier
function buddydev_do_not_notify_join_group_activity( $stop_notification, $activity ) {
    if ( $activity->type == 'joined_group' ) {
       $stop_notification = true;
    }

    return $stop_notification;
}
add_filter( 'bp_local_group_notifier_skip_notification', 'buddydev_do_not_notify_join_group_activity', 10, 2 );