A dromedary camel stores fat, not water, in its hump.
The hump is a concentrated energy reserve. When food is scarce, the camel metabolizes stored fat, producing energy and some metabolic water. As the reserve is used, the hump can become smaller and less upright.
This adaptation helps dromedaries survive hot, dry environments where food and drinking water may be widely separated. Their other adaptations include efficient kidneys, tolerance of substantial body-temperature changes, and nostrils that can partly close during sandstorms.
The familiar claim that camel humps hold water is incorrect. Camels do drink large quantities when water becomes available, but that water is distributed through their body rather than stored in the hump. Dromedaries have one hump; Bactrian camels have two.