Why do presidential powers tend to grow during national emergencies?

The government doesn't really gain anymore power than it already had, the president before stepping into office has to swear an oath saying he will do anything in his power to protect the Constitution, and being that it is a government it was created to serve we the people of the United States so really the President is really gaining more power over our say to do what he must to keep a balance of power, otherwise, we could go to war against ourselves if we just assumed the President was running without support of power.