Reason 1 - it is one of the simplest ways of tailoring your CV to the role you are applying for, thus increasing your chance of selection for an interview.
Reason 2 - Including Achievements is an excellent way of expressing what motivates you. It helps to include metrics and measurable that are specific to you.
Reason 3 - Including Achievements/accomplishments in a CV helps to express the difference your employment has made to the organisation.