When you're applying for a job, you want to put your best foot forward. You want the potential employer to know about all the things you can do that are relevant to what (s)he is looking for.
If you're a student or a recent graduate of a hands-on program like CreComm, much of your "evidence" is in the form of assignments completed in class, or on internships, or as a volunteer -- and you need to ensure you make it clear what was volunteer work what was paid.
What difference does it make?
A hiring manager reads more into the work experience you describe in your cover letter (and which you share in your portfolio) than the quality of the work: (s)he also makes inferences about how other professional communicators may value your work.
For example, if you say you created a media kit for a Red River College corporate announcement (which you did, for a fictional scenario in an assignment), you are technically telling the truth. But the statement is misleading: you are inadvertently suggesting that Red River College actually used your media kit, which implies that it met Red River College's corporate communications standards. It may have been of a high enough quality for the college to have used it -- but the college didn't use it. You don't want your cover letter to inadvertently imply that it did, and take credit that isn't due.
Similarly, work you do as a volunteer for an organization is much appreciated -- but it isn't paid for. Your hiring employer won't think your work any less valuable upon seeing that it was done as a volunteer (in fact, (s)he might think more of you for offering your skills and energy to a deserving client free of charge), but it's important (s)he doesn't think you did the work as a paid consultant. Again, that could lead to assumptions about how the client valued the work -- which, again, could be true, but you don't know it to be. So you shouldn't leave the door open for the employer to assume it.
What to say
A hiring manager looking at your resume will (or should, if you've written it well) know exactly how much professional experience you have or haven't had. If the organization is looking to hire someone right out of school, it's expecting candidates to have work samples that originated in school assignments or volunteer work -- there's no reason not to confirm it.
Further, if you do have work samples representing paid work you've done, either as an employee or on a freelance basis, make sure you point that out too. All that information comes together to give the hiring manager a complete picture of what you bring to the table.
In being clear and up-front about the nature of your work samples, you signal to the employer that you are open and honest, and proud of your work... and those attractive personal qualities will shine through your cover letter, along with your skills and experience.