Since the fasting blood suger (or non-fasting for that matter) that your meter gives you, can spike or dip any number of times during the day, a running average over the last 90 days is what the gA1C gives you. I use it as well as the meter but relying on the A1C means I don't have to test many times per day. Once is enough. Several times per week gives me an acurate picture and the A1C gives my doctor a good tool.
Though, knowing she'll ask for my daily number from the meter, it keeps me honest and doesn't allow me to 'game' the system. Without daily meter tests, the A1C can be misleading.