Requirements are prioritized in order to document their relative importance to stakeholders. This is a very important step in the Requirements Life Cycle Management knowledge area because when the requirements of a project have been prioritized, you can determine the order in which they will need to be delivered. It will also aid in understanding the value that the stakeholders attach to each of the requirements. 

Dependencies between requirements can also often assist in the prioritization of requirements.

The prioritization of requirements is not a one-off activity but rather an ongoing activity that needs to be closely monitored and actively managed by the business analyst in close collaboration with all other stakeholders.