1. You are good at mathematics. Especially, if you want a finance ph.d, i am talking linear algebra, differential equations and brownian motion. if you have a math or physics or engineering major, you will be a strong candidate. Plus, if you are good at math, rather than spending all your time learning math, you will spend more time learning about applications of math to your research. You might be able to complete your research faster and publish it.
2. Research Experience. if you completed a masters thesis in your masters, you will atleast have a faint idea about what it would take to do a original piece of research. if you dont have research experience, it wont hurt too much, but you will need to study some papers in your field to get a better idea of a good dissertation.
3. You CAN survive on the stipend for atleast 4 years. this is a big question for most students. if you are married and your spouse is earning , it is a big relief. On the other hand, schools generally provide generous stipends. Some schools will offer teaching appointments in the third or fourth year and that brings in more money. the bottom line: it will help if you have some savings before you start your Ph.D.
These are only some of the Top reasons. feel free to add to my list