If you have a lot of contributing authors whose posts you want to be able to credit and list in archives, but you don't need or can't afford the time it takes to to create separate user accounts for them all, then you need this plugin. It will also handle the problem posed by multiple authors/co-authors.
By separating bylines from the author/user who created/owns the post, this is the first simple solution to a longstanding problem for multi-author WordPress sites. You simply add any number of author names to a "Bylines" taxonomy (like tags), and they will be automatically linked to their own taxonomy archive page whose description can be used for a short author bio, and the Taxonomy Images plugin can be used to assign them photos.
Byline is much simpler than implementing Co-Authors Plus, which is often more solution than you need and still requires creating user accounts for authors if you want to link bylines to author pages.
The main comparable alternative to Byline is Custom Author Bylines. Unfortunately Custom Author Bylines is buggy, harder to implement in some themes, and unsupported.