Why are reserved powers important?

The main reason why reserved powers are important is that the Constitution was meant to evolve with the times, so without reserved powers the nation would come to a grinding halt due to the fact that everything would need to be written down. 

Opposition to the nra came to a head in 1935, when the conservative supreme court declared the agency unconstitutional, claiming the nra gave the executive branch regulatory powers that belonged exclusively to congress. this decision began a series of court decisions that overturned key elements of the new deal.