It's obvious advice to tell writers to write as much as they possibly can. It's less obvious advice to tell writers to put their work out in the public domain--even if their work isn't quite ready.
Writers don't just need to learn to write. They need to learn how to deal with agents, magazines and online journals. They need to learn what kind of writing is in demand, and what kind of writing is not in demand. The only way to learn this is by putting your work out there, handling the inevitable criticism and rejection, and then adapting to the feedback.
That's why, as soon as you've written some articles, fiction, or essays you're happy with, submit them to a small circle of smaller magazines, newspapers or online sites. Get started. Use the process to get familiar with each publication's submission and editing process. Learn what the market potential really is for your writing. Take whatever feedback you can get, learn from it, and adjust accordingly.