Why are gene duplication and exon duplication and shuffling important for the evolution of proteins?

Duplication of an existing area is one of the methods that new "space" in a genome can be acquired.  Not only this, but as the gene is for a working protein and not simply genetic "noise", it will already have the appropriate start/stop codons for transcription.  

Basically, because the protein is being copied, modifications to the copy will not, generally, hinder the organism's normal mechanics since the copied protein still exists.  Modifications via point mutations of the copy may result in a new protein.  If it has a positive influence on the organisms ability to survive and reproduce, the protein will be spread throughout the future population due to an advantage over other individuals that do not have it.