You know how you can type in a few different versions of a URL and you still end up in same place? For example, you might type in www.websitename.com, or websitename.com, or http://www.websitename.com, or even www.websitename.com/home.html. All four of these options will get you to the website homepage that you are looking for, and this is great functionality for the user. But those are essentially four different URLs, all with duplicate content. Duplicate content can harm your ranking score on search engines, so you want to avoid duplicate content where possible.
Another issue arises when the user doesn't type in the URL, but searches for your industry or product or brand using a search engine. In the example above, the search engine gets to pick which URL its going to display. And that might not be the one you want to rank, thereby diluting the authority and backlinks leading to your site (another issue!).
A way around these issues is using canonical tags. Essentially it's telling search engines which of the four URLs is your preferred customer-facing piece of content, so it directs all four options to the single most dominant, or your preferred option. This is similar to, but definitely not the same as, a redirect. Using canonical tags is an important part of an effective SEO strategy.