After a heavy night of drinking your body will expel huge amounts of water, electrolytes and nutrients from your body. This results in your bodies blood sugar levels dropping dramatically, which is what causes you to feel sweaty and even shaky after a taking plenty of drinks the night before.
If your stomach is up to it you often crave a huge breakfast the morning after the night before so that you can replenish those blood sugar levels and give your body fuel to expel all the alcohol from your body! If you don't replenish those blood sugar levels then you will continue to feel tired and irritable, and no one wants that.
It is recommended to eat a few pieces of toast because the carbohydrates which it contains will assist greatly in increasing the blood sugar levels in your body slowly and bring them back up to normal. Plus the big bonus is that even if you are having stomach trouble the toast is easy on your stomach and should not cause you any trouble as it alleviates nausea and helps your body soak up the remaining alcohol in your system.
Plus we all know that a nice slice of toast is extremely tasty too!