Hiring a partner to do your enterprise SEO for you means that you can focus on your business and let a knowledgeable, experienced agency take care of your SEO for you.
SEO requires a lot of daily updates and groundwork, a strategy that's able to adapt to your current performance and fluctuations in the algorithm, as well as constant combing through pages of data and analytics.
It's best to leave your enterprise SEO to a company like Fishtank who work with enterprise SEO day in, and day out, so you can focus on other aspects of your business and know that your SEO is being taken care of for you.