To prevent a man in the middle attack. There's a possibility that a hacker could inject malicious scripts into the payload, or even completely change the content of the website that you think you are viewing.

HTTPS stops the content being modified on its way to you. Some less than scrupulous ISPs have injected their own ads into pages, which would be prevented with SSL.

Someone sitting between you and the site (Wifi router? ISP?) could potentially inject their own scripts.

Thanks. All pretty valid arguments, and probably enough to outweigh any potential slight performance/latency issue.