This plugin allows you to write e.g. shopping cart code
which should behave well for guests as well as permanent users.

The basic idea is both logged in and not logged in users can
get the same benefits from sessions where it doesn't matter,
but that logged in users can keep their sessions accross logins,
and will even get the data they added/changed assimilated to their
permanent account if they made the changes as guests and then logged in.

This is probably most useful for e-commerce sites, where the
shopping cart is typically used before login, and should be
equally accessible to both guests and logged in users.