Traditional publishing agreements typically transfer all rights, including full copyright, to the journal. You probably want to include sections of your article in later works. You might want to give copies to your class or distribute it among colleagues. And you likely want to place it on your website or in a repository. These are all ways to give your research wide exposure and fulfill your goals as a scholar, but they are prevented by traditional agreements.
The SPARC Author Addendum is a way to negotiate your publishing agreement and retain your author's rights.
How to use the SPARC Author Addendum:
What if the publisher rejects the author addendum?