Biology question regarding systolic and diastolic blood pressure.?

Fluid flows from high pressure to low pressure. Thus the difference between systolic and diastolic pressures is critical in blood flow. You want to enhance the difference between these two when high flow is needed, and that is exactly what happens. If the diastolic pressure were increased, the heart would have to work against that to get the blood to flow, which is one reason excessively high blood pressure is detrimental.