There are three types of "Emergency" in the French system. The first two are constitutional, the third is statutory law, and it was the third type that was used in 2016/17.
The first can be declared when the territory, people or constitution of France are in grave danger. The President can take emergency actions to protect France. But the "Emergency" can only last for a maximum of 30 days, unless extended.
The second type is a "seige of the state": a peril resulting from a foreign or civil war. It allows the military to take police powers, limited to 12 days, though it may be extended by parliament.
The third is a law that predates the current constitution. It allows the President to declare an emergency giving exceptional powers to ministers and prefects to order house arrest, curfew and other such provisions. The intent is to allow civil authorities to manage an emergency without giving the military wide powers, or declaring a "seige". Again it is limited to 12 days, though it may be extended by parliament. In such an emergency the Constition of France is still in full force.
These measures exist as the constitution is intended to slow down decision making to allow for democratic consideration. However, if the wolf is at the door, this slowness can be harmful to the French. The overriding purpose of the constitution is to protect the interests of the French People. In normal circumstances, it does this by a representative democratic form of government. When this is not possible it allows for the interests of the French people to be protected by other means.