Most people schedule their workout early in the morning before they start their day, partly because it forces them to get up and do it before they have the time to talk themselves out of it. Exercise any time of the day is good exercise, but there actually are some advantages to those early morning starts.
Empty stomach. Your body release insulin after eating, and insulin may interfere with driving your body fats and energy. After fasting for eight hours through your sleep your body will have already digested all of your foods and any immediate energy. Your body will then be forced to feed off of anything extra your body has stored, and use any excess fats lingering around for energy.
Your metabolism will utilize your workout more effectively through out the day. When you exercise your body continues to burn calories for some time even after your workout is over. If you workout in the morning, your body has the rest of the day to continue to burn. Evening workouts don't allow you to get as much burn because your metabolic rates drop so much as soon as you go to sleep. But if your schedule only allows you time in the evening, don't let these facts make it not worthwhile, because you are still getting your cardio time in somewhere. You are still improving your health at whatever time of day it is.