Why do defects have priority and severity?

Defect priority indicates how urgent it is to fixing a defect. This is to help developers and testers identify and prioritise which defects to focus on when it comes to closing defects. If a defect is deemed to be a low priority then this bug may be deferred depending on budget and time constraints of the project.
Defect severity describes the degree of impact of the defect on the system should this defect not be fixed.