One of the most important questions a PhD applicant will be asked is "What are your research interests?". So, you should be prepared to answer that, even if you don't have a definitive answer.
Actually, it is a little strange if an applicant already has clear-cut research interests, since those interests will probably evolve with time, specially during the PhD itself. But you will need to have a general idea about what subjects make you tick, and explain why.
I believe knowing that is an essencial part of applying for a PhD. One reason is that you should have some interest that will motivate you for several years, maybe for the rest of your life. It cannot be just a curiosity, or a passing fad. It will be hard to keep researching something that doesn't interest you anymore. Another reason is that knowing you interests will help you to find the universities, PhD programs, and professors who are best fitted to help you in your research.
In my case, my research interests can be summarized as "to discover and develop measurements for Marketing results, specially metrics integrated with Finance". So, I want to research about the best ways a company can measure results from its marketing activities, with a special interest in the financial point of view of those metrics.
Even though I applied for Marketing PhD programs, my life prior to the PhD was one focused on Finance. So, what made me become more interested in researching about Marketing than Finance?
While working with Finance, one of the biggest challenges was to analyze and justify marketing investments and budgets. I worked for a company, for example, whose Marketing investments amounted to tens of millions of dollars a year, in several projects, including music concerts sponsorhip, its own stock car racing team, and a top model as pitchwoman. How do I know if the results from each of those investments were really worth the money?
The goal of the financial management in a company is to maximize the company's value to shareholders or stakeholders. And it's probably common sense now that one of the ways to do that is to maximize value to the company's consumers.
So, I don't see Marketing and Finance as completely separate things, I believe both should be taken into consideration together when managing a company. But there is much to be found yet about how to do that effectively. There are much more questions than answers. That's where research can contribute. I hope, with my research, to be able to help companies to find ways to improve their results to both shareholders and consumers.
This is an interesting article about value maximization, for those interested in the subject: Value Maximization and Stakeholder Theory