Hijackers sell the same product and piggyback on your product. Hijackers are notorious on Amazon. They can harm your brand in various ways: stealing sales, taking the buy box away from you, undercutting you, selling damaged or counterfeit products to hurt your reputation, etc. Having your products in the Brand Registry can better protect you because you will also have a trademark on your products.
Helium 10’s Alerts will monitor your ASINs 24/7 and help you stay proactive against hijackers.