Properly reading an assignment helps students schedule their study time because it allows them to know exactly what they have to do and when they have to have it done.
If you do not read an assignment properly, you may not clearly understand what is expected of you. You may not notice things like how long a paper must be or that you are supposed to hand in a rough draft of the paper. You may not notice what the deadlines are for the various parts of the assignment. If you do not know exactly what is due and when it is due you will not be able to plan properly because you will not know exactly what tasks you face.
If you do not read an assignment properly and do something that you were not meant to do, once you complete the whole assignment you realize you wasted not only your time but also your energy.