You should create a portfolio on crowdspring because it will show off all your design skills and great talent to the world. As an added bonus, clients will be able to see your past work. This can often influence which creatives they choose to participate (and occasionally award) in their projects. And of course, a client will not invite you to a 1-to-1 project unless they can take a look at your portfolio first.
And remember, only creatives who have good portfolios on crowdspring are eligible to participate in Elite and Platinum projects (assuming their reputation scores also qualify them).