Every time you talk to prospective clients, they'll want to know why they should trust you.
Remember, it's not just money. If someone does a bad job for them, they might not have the time to get it done right. So, the stakes are high. Your past work is one of the strongest ways to show that you're right for the job. Collecting it in a portfolio is one way to convey this information.
Once you have all the pieces in one place, you need a way to display them. Nowadays, the usual place is a portfolio website. If your potential clients live more in the off-line world, or if you expect to meet a lot of them face to face, you might also want to have a printed version of your portfolio. I've assumed that you have work to show.
What if you're trying to freelance in an area where you don't yet? Frankly, I'd recommend you reconsider your choice because the lack of a portfolio is really going to hamper your efforts. One other option isto plan to start with forms of marketing that don't require a portfolio. For example, advertising.
Another is to do some jobs for low pay or even for no pay to build up your portfolio. This is a good opportunity to do favors for family, friends and non-profit organizations that you support.
Finally, it's a good idea to create two other pieces to complement your portfolio. The first is a brief text that summarizes your experience, maybe a hundred words or so. You'll use that in e-mails, applications and marketing materials. Eventually, you'll have several versions of it for various purposes. I personally keep a plain text file on my computer so that they're always at hand. You'll also want to create a resume. It'd be nice if others could intuitively sense that you're right for a job, but they can't. They need to be shown and nothing convinces as well as a clearly presented record of success.
That's what a well- prepared portfolio does for you.