In what ways would co-authoring a graduate research paper as an undergraduate help me?
It doesn't exactly "allow you to get a Ph.D quicker". However, you would have more experience in academic writing than if you hadn't contributed to the writing in the paper. Writing a paper isn't just "writing" a report - a lot of analysis, interpretation, and technical work goes into papers and these are skills you want to have as a Ph.D student. Being a co-author on a paper will make your application stand out.

If you go into any job, you can list this as a project that you have worked on. If you are familiar with the contents of the paper, then you can talk about it as if it was another project that you've worked on. Having your name on the paper is verifiable and can look pretty impressive especially if you are familiar with the details.