The purpose of these positions is primarily to train people who are interested in the job, at little cost to the employer because most internships are unpaid. They're supposedly a good way for people to gain valuable lessons in a job market where it's almost impossible to land a job without some form of experience.
Many recent articles in the media have decried this practice as companies trolling for "free work". I could easily agree, seeing as I've been interning as a writer for an online magazine for the past six months and there has been no indication from anyone as to whether I'll be paid for my work any time soon, or even if they'll recommend me for a paid position elsewhere.
I'm registered on at least a dozen online job-search portals and I keep my profiles updated regularly. Fully half of the "Recommended jobs for you" that they show me are unpaid internships. Why? I am not a student or a recent graduate. I've fourteen years of office experience, half of those as a writer and editor.
The search engines themselves are partly to blame. They use key words to find matches based on what's in a person's profile. Early on I kept receiving job listings for Project Managers simply because I had the word "managed" in my online resume. Once I reworded the appropriate section, the listings disappeared.
Perhaps now the search engines think that an internship would be a good fit for me because I have a sizable gap in my work experience due to the fact that I stopped working to raise a family. Granted, it is a good way to get one's feet wet prior to returning to the workforce, but I feel I'm worth more than that.
It's like the old paradox: can't get a job without experience but can't get experience without a job.