Its to try to mitigate against a (generally state) adversary saving the session and waiting 10 years or more and then breaking the now weak encryption (ie rsa-2

Its to try to mitigate against a (generally state) adversary saving the session and waiting 10 years or more and then breaking the now weak encryption (ie rsa-2046 is projected to be "usable" until 2030 by NIST) This is one of the reasons why the NSA (and other intelligence agencies around the world) have these huge data centers that essentially just store everything they collect. So that later they can retroactively go through and get a back story.