To begin with, a little of everything. If you can write interestingly and with freshness and insight in a related field, you can learn to apply that writing skill in a business situation.
Where many copywriters (and art directors, etc.) are lacking is in the business aspect.
Your writing needs to speak to the real needs and emotions of the people who buy what the agency's clients are selling. That's true whether you're selling cola to the masses, or computer servers to a small number of engineers. You need to know your market, and be able to talk with them as if you were talking to them personally.