Every time I hard boil my eggs (which is often), I cool the water and use to water my indoor and outdoor plants. I have learned the extra calcium from the egg water is very good for plants. Why waste the water by pouring it down the drain? I hard boil eggs several times a week for breakfast, egg salad, to put into tuna fish salad and also to make deviled eggs.
Source: A friend shared this with me and I have been doing it for years.