Imagine if you are in a coffee shop and using public wifi. A hacker is also in the same coffee shop on the same wifi. You are purchasing something off of an insecure website. Little do you know that the hacker on the same wifi network is monitoring your traffic with a packet sniffer(a tool to monitor your doings online).
Without the Strict-Transport-Security Security Header, your transaction is not secure. The security header tells your browsers to initiate a secure connection.
