I love this plugin, Enforce www. Preference. I actually have looking for this plugin since I know about SEO and concern about being penalized by search engines. By redirecting your website from non www to www prefix, it will increases your chance to get higher place in SERP.
The idea is simple. For an example, click on this link.
Even the url above not has www prefix, the plugin will redirect it to the same article but with www prefix as the link below.
If you still confuse, search engines will penalized website which have same content even the articles hosted on the same domain. But, search engines treat those two urls as different domains. That is why you need to use this plugin to redirect your articles url to only one address.