When states declare a state of emergency, they essentially waive many normal legal procedures and requirements. This enables the government to more quickly step in and help affected communities and citizens. It also empowers the government to effectively inact temporary laws that might otherwise not be permitted or violate existing laws (e.g., bypass competitive bidding for contractors to build/repair important infrastructure that would normally require a lengthy bidding process, see the i85 bridge collapse in Atlanta).
The purpose of this is obviously to provide immediate relief to communities suffering from a disaster or crisis. This can include things like shutting down transportation, suspending the Clean Air Act, and seizing Americans’ private property.
However, states often invoke the state of emergency for reasons that have little to do with actual emergencies and are designed to give the executive branch additional powers — which can lead to egregious violations of civil rights and liberties. As the Brennan Center for Justice has documented, the declaration of a national emergency gives the president over 130 discrete statutory powers that allow him to suspend or limit a range of constitutional protections.
It is not unusual for the president to invoke the state of emergency in the face of a natural disaster or public health crisis – it has happened during Hurricane Katrina, in response to the cholera epidemic in Haiti and to wildfires in California. It has even been used by a democratically elected leader to circumvent the normal legislative process and take military action against a foreign adversary.