I wanted to know why some eye doctors dilate the eye and some don't?

Different reasons. Dilation is usually performed on an initial examination, annual exams and if a doctor is concerned about any conditions related to the back of the eye. Some doctors use photography through undilated eyes in order to avoid this step for healthy eye exams, but the image quality does not compare to a good dilated exam.
Depends. See previous answer. Depends what they are looking for and and attempting to accomplish. At times they can see what they need to see with the normal pupil. At other times, a wide-open pupil is necessary.