Jalandharbandha is very useful for Pranyam. Kumbhaka (breath holding) is not allowed without Jalandharbandha.
Neck and throat is constricted. And the movement of air is stopped. Air remains inside the chest.
According Hathpradipika it is said to protect from Jara (old age) and Mritu (death). It helps for the health of upper respiratory tract, heart, head, brain and neck.