Many websites have both the traditional, unencrypted HTTP and the SSL or TLS-encrypted HTTPS addresses available to access their content. Wikipedia is one good example of this functionality. You can easily view Wikipedia using traditional HTTP protocol but if you wanted or needed a little more privacy the HTTPS address is available as well. Unfortunately it is sometimes hard to know if a website has the encrypted feature or not unless you try. And you might be in a hurry and forget to use the HTTPS version and then you've potentially sent sensitive information about yourself out onto the Internet unexpectedly.
There is an easier way, however, to use HTTPS whenever possible. The Electronic Freedom Foundation (EFF) has released a plug-in for Firefox and Chrome that knows of almost all of the commonly used websites that are available over HTTPS and will dynamically redirect your web browser to use that encrypted channel without you having to remember. The plug-in, known as HTTPS Everywhere, will convert any web address from HTTP to HTTPS whenever it knows that HTTPS is available.
Why is it important to encrypt your traffic whenever possible? Well, simply you never know who might be listening to your connection. If you are living in a country dominated by an oppressive government then your liberty or even your life might dictate that you need to obtain your information via encrypted means. Other people might be more concerned with their private browsing getting into the hands of a corporation to be sold to the highest bidder to get more information on you into their files. Others are just concerned with their privacy in general. Whatever the reason it's a good idea to use encryption whenever possible.
It should be noted that HTTPS Everywhere doesn't automatically encrypt all websites and users should still verify that the lock is showing in the browser address bar and that the certificate matches the site in which they are visiting. That said, using encryption makes your Internet browsing safer and this tool makes it easier.