[AnthonyMouse]

Suppose you have a MITM attacker, e.g. hotel WiFi. You have any page not using TLS open in a background tab, which the attacker uses to inject javascript. Meanwhile there is a different page open via TLS which you're actively using, so your browser is constantly using the session key to encrypt the traffic. The attacker is now recording the encrypted session and after an hour they crack the session key and can use it to go back and decrypt the traffic.