Why Should Authors Consider a Book Marketing Agency in Today’s Crowded Market?

I’m curious why so many authors are turning to a book marketing agency instead of handling promotions on their own. What specific advantages do these agencies offer, and which strategies actually make a measurable impact on book visibility? Also, who benefits most new writers or seasoned authors? With so many promotional options available today, what makes a professional agency worth choosing, and when is the right time to seek one?