This post represents a deliberate misrepresentation of the nature of national emergencies declared under the National Emergencies Act.
The Act empowers the President to activate special powers during a crisis but imposes certain procedural formalities when invoking such powers. The perceived need for the law arose from the scope and number of laws granting special powers to the executive in times of national emergency. Congress can undo an emergency declaration with a joint resolution. Powers available under this Act are limited to the 136 emergency powers Congress has defined by law.
The Act was passed because of Vietnam, basically; it gave Congress the ability to end a national emergency declared by the president with a joint resolution. Prior to the Act presidents could declare national emergencies but Congress couldn't do anything to end them; after the act all emergencies were reset to zero and Congress gained the ability to reign in any presidential resolution that had gone on for too long. "National emergencies" are things that are declared without congressional oversight because, in theory, there's no time for Congress to act; in practice, national emergencies also include things that Congress doesn't oppose universally enough to strike down via joint resolution.
The "national emergency" related to the Iran Hostage Crisis is about asset seizure. Every year, we have to decide if we're still mad at Iran. Every year, we are. The "national emergency" related to WMDs is about executive privilege and reconnaissance of the world's nuclear stockpiles. There's a massive facility in my hometown that converts warheads to fuel rods that you can't see on Google Maps governed by this declaration and the principle reason it's within the Executive is compartmentalization. The "national emergency" related to Cuba is due to a Cuban shoot-down of an American civil aircraft and the declaration expands US territorial waters. Are we still mad at Cuba? Why yes. Yes we are. And we reserve the right to patrol the waters between us and Cuba so long as Cuban MiG-29s threaten American Cessna Skymasters.
The "national emergency" related to Trump's border wall comes after any attempt to secure funding for it failed through normal channels. It is therefore de facto not an emergency, which is why everyone with any sense of the constitution is pissed off.