People overlook the fact that it is very easy to capture your data over wifi. ANY user on the same wifi network as you can capture your packets as it travels to/from the access point. A lot of tools out there allow amateur hackers to do this.
SSL encrypts the data, so even though the person next to you in the coffee shop can be capturing and storing your data, it's encrypted thanks to SSL and is basically useless to the hacker. Without SSL, your username, passwords, credit card info etc is plain text and is as easy to read as this post.
Also, when your site has SSL, people trust it and are willing to provide sensitive data like password and creard info. Because of this ecommerces sites by and large use SSL on their sites.