The larger the portfolio of work you have, the better the chance you have of hitting a constantly moving target. To give themselves the best possible chance, writers should build up a formidable arsenal of work to compete in an already tough industry.
Look at it from an agent’s and manager’s point of view. They’re unlikely to take on a client who doesn’t have much in the way of writing samples or specs to their name. They want clients who put in blood, sweat and tears and who successfully churn out work at a constant rate. And who will ultimately make them money. But don’t forget, as a writer you need to do a lot of ground work first.
Give yourself the best possible chance.
Build that portfolio.
Because they'll more than likely put this question to you.
What else do you have?