The career portfolio is important for several reasons.
First, your online portfolio showcases your ability to design and maintain websites, a useful skill in any job. The more you can promote yourself appropriately on social media, the more you will be able to land jobs requiring familiarity with social media.
Next, labor is increasingly moving from a model of a single lifetime job, for which employers are interesting in hiring and developing permanent employees with versatile skill sets to a contractual or freelance model in which employers contract out work to independent contractors. As an independent contractor, you are hired on the basis of using a portfolio to demonstrate that you can successfully complete a specific project by showing examples of similar work you have done in the past.
Finally, as increasing number of people compete for jobs, a portfolio (especially an online one) gives an employer a chance to look over your work very efficiently and showcases your abilities in a way that makes you stand out from the crowd.