Registering you beacon will speed up the rescue authorities attempt to rescue you, providing valuable information to help identify you and contact people who might aid in identification. It is very important to ensure your registration details are accurate.
This US produced brochure describes the reasons for registration in more detail.
Please note that normally the registration should be in the country of residence, or of the vessel’s registration. When you purchase an EPIRB or PLB from a supplier in another country, make sure they configure it for your intended country of registration, otherwise it is unlikely that the registration will be accepted.