Many areas of the arts will require you to present a portfolio in a job interview. If you’re a writer, your employer needs to know you can actually write, and write well. Similarly, if you’re an artist, you will often need to demonstrate that you can create good work. On the other hand, your portfolio might consist of business plans you created for the last organization you worked for, or designs and reports you might have made in the past.
Either way, a portfolio is a chance for you to demonstrate your accomplishments in a much more effective way than just presenting a resume. Even if you are great at what you do, having a specific example of how you work and what you produce might hint to an employer that you are just the person they would like to work with.