Agreed with the above answer. Everywhere in the world you find poverty, you also find prostitution. There is a direct correlation between the amount of economic opportunity in a society and the amount of prostitution.
While there are certainly those who make a very good living on the world's oldest profession, the vast majority of women do not, and use it to support themselves, their families, or their addictions. Many are involved in endless cycles of violence in this profession.
Also, in the Philippines, there are fewer laws to prevent human trafficking, or at least they are not well enforced. So the Philippines may have more than its share of this problem.
I would think that the reason for prostitution in the Philippines would be the same reasons the trade exists all over the world. It seems to me that women who enter prostitution do so for lack of a choice. On some level, these are women who are poor or do not enjoy the benefits of upper class or elevated social strata. They are in need of money or have to support other family members and prostitution is something that is something which can meet this need. There are lines of logic which argue an ultra feminist view of prostitution. Thinkers like Camille Paglia argue that prostitution asserts the strength and power of women, as they are in control of their bodies and "in control of the streets." Yet, there are an equal number of advocates who argue that the flesh trade and prostitution are male controlled industries where women are used as means to an end, as opposed to an end in of themselves.