SSL is used to secure the data in transit between server and client. It is to prevent anyone from reading the traffic or modifying it.
The risk would be that a hacker who has access to the persons WiFi connection could change the Bitcoin address displayed on the website and replace it with his own, allowing him to steal Bitcoins. They could also read any passwords etc This could also be done by the owners of a VPN/proxy service the user was using, or a hacker who has compromised an ISP that is transferring the packets, or an intelligence agency that has compromised a backbone internet router.
You should always use SSL when you can even if you do not think it is important, as some users may not want the NSA being able to easily spy on their traffic to your website. you can get a PositiveSSL cert for $2, and soon the EFF plan to launch a CA that will provide free SSL certs:
https://letsencrypt.org/
