It is very important for those who are planning to come in the USA to pursue PhD.
When you write Statement of Purpose (SOP) for a PhD program it has to be very specific. In fact, you should study a particular university and their research programs very carefully and then you are supposed to write in your SOP that you are really interested in their such and such program and that's why you are writing to them. In a PhD program they expect you to have a very clear idea of your research interest.
PhD is a research program, so try to convince them you have a real research interest mentioning your publications etc.
Best of luck with your higher studies.
Writing courtesy:
Harun or Rashid
Assistant Professor
Department of Physics, BUTEX