For example, I did a local trail race on Saturday. Nothing near what my Main Summer races will be, but it was right near my house and I had a hard run workout scheduled for the day so I said "Why Not?". Well sure enough, I learned something. A big Something.
My Heart Rate at the start of the race was 20 beats faster than the normal start of any workout I do. My Max HR during this race was 13 beats higher than my max during any of my workouts. What does this mean? My body was just introduced to a new variable and unfortunately- it happened during a race. This is what you don't want to happen during your Main race.
I know a lot of people running the Boston Marathon. They could be in for a long day if they have not prepped for the first 3 hours before the race where you are just sitting in a grass field with no shelter in what could be some frigid temps. This doesn't mean they should do that exact thing on a training run, but I would highly recommend them being awake 5 hrs before one Long run to make sure their body is use to that. Does that make sense?
Don't Let the day of your Main ("A") Race be the first time your body experienced something. Push your body during those training workouts (a select few) or do local races so you know (somewhat know) what to expect on the Big day of your Main Race.
Oh, and even cooler- as I always do with endurance sports- I related this to life. Why not go through trial runs with events in your life before just jumping in? It might work out it might not but at least you wont be surprised in the end.