You don't need to use a marketing agency, you could do it yourself. But they have the expertise and tools that you don't have, so you can spend that time working on what your business core activities are.
Look at it this way:
I could paint my house, but it would take me longer than if I hire a painter to do it for me. Besides I can keep on working on my stuff while the painter does his job, so I make more than enough money to pay him afterwards.