There are a number of reasons for you to write the GRE revised general test. Firstly GRE is the only Test that is accepted by graduate schools and B-schools. No other tests including GMAT can match GRE in this regard as GRE went through a metamorphosis to compete with GMAT. GRE is accepted in almost in all graduate programs.
Secondly GRE general test tests your reasoning and analytical skills rather than your subject knowledge which is normally tested by your undergraduate or graduate exams. Thus ETS provides graduate schools an additional parameter to look upon.
Graduate schools receive applications from thousands of different students who attended theirs undergraduate or graduate programs in different countries and different universities within the same countries. This makes comparison almost impossible. The GRE score provide a common platform for most these schools to make reliable comparison between their applicants.