I am not familiar with the field of accounting. However, the main (but not only) reason to get a biz phd is to be an academia. Either on a research or teaching track. But basically, to stay in school.
If your objective is to be a CPA (or some other professional certification), there is no need to get a phd. You will be much better off focusing on the CPA requirements now, than to spend the next 4 to 5 years as a phd student.
And since you are still in school, talk to you advisor or some profs to find out more. Find out what's involved in getting a phd. Also, they will be in the best position to advise you on your research topic inquiry. You are in position that I hope I am in.