Originally, NPIP started off when the postal service started shipping chicks through the mail--this started the spread of s. pullourm across the country and was devistating to the poultry industry. Pullourm is almost 100% fatal. When you get your flock NPIP certified they are testing your birds for s. pullourm in their blood.
Today, NPIP has expaned their research/influence in the poultry industry to include diseases such as Avian flu, merecks, etc.
"responsible" breeders and sellers should have their flock NPIP certified. And actually, it's "illegal" to ship birds/eggs out of state without having NPIP paperwork