Freelance copywriters have to both sell accounts and collect payments, in addition to doing the project work, as do all freelancers.
Perhaps you've realized that you have a way with words when it comes to selling things or distributing ideas.
Perhaps you're creative at heart, but your talent lies in the verbal.
Some advantages to career copywriting work versus freelance work are present.
For example, ad agencies tend to attract much bigger clients (think name brands/household names) than a freelance copywriter will.