There are several important reasons for writing articles as a freelance writer. This is especially true when you are first attempting to get your work published. 
In the beginning, you may think that you only want to write short stories or editorial pieces. However, if you do specialize in a particular genre, you will quickly find out how difficult it is to market your work. Article writing is what you want to concentrate on as a freelance writer when you begin, as you will have a far better chance of earning through article writing.
One of the benefits of writing articles is that you will see over time how it helps improve your basic writing skills. It will also help you get used to using the correct formats and making sure you are scrupulous at proofreading and editing, whether you are submitting to magazines or online sites.
Another important reason to write articles is that the more you write, the more confidence you will have as your writing improves. Building confidence is something you need as a writer, as you will be submitting articles and perhaps getting a lot of them rejected, which is natural for even experienced writers.
Article writing also teaches you how to write on various subjects. For example, you may write an article on how to plant hydrangeas. Your next might be on the reasons for Iceland's financial meltdown. You get the idea. The more you practice writing about different things, the wider your scope will be; and soon you will be able to put together an article on just about anything without breaking a sweat.