If you are a graphic designer, whether that be for print work or website work, it is important to have a solid portfolio of your work to show potential customers what you can produce. It may be that you have no qualifications relating to design work and therefore the only thing that potential customers have to go off is your previous works.
You can rave all you want about how good you are at graphic design, but examples of your work are the best-selling tool you can have. Ideally when creating a portfolio, talk about what the brief was for the project and how you met that by the design you created.
IF you are redesigning a logo for a client then creating stationery, have a copy of their old logo and then the new one. Discuss why they wanted to change it, then state what you did (that is within the brief) that created the final artwork.
Try to keep your portfolio up to date and archive case studies that become to old and may now appear outdated. Your potential clients may even want to speak to some of your clients for a reference, so it is always a good idea to have permission to pass on details of a few of your clients that may be happy to be contacted.