How and why do we tag the complexity and severity to a defect?

Prioritizing defects by assigning complexity and severity is good for defect management.

Not all defects need to be rectified immediately and not all defects affect the proper function of software, there might be walk around to avoid certain defect before it is rectified.

Prioritized defect will help developers to focus on the urgent defect which hinder the quality of software within schedule timeline, and also for proper planning.