What was the purpose of world war 1?

Sorry for the late answer, but hope it helps!

Well, the purpose is hard to tell, as each of the participated countries had different reasons for joining in the war. Generally you could say that each of the major players wanted to either strenghen their international position and power or stop their empires from falling apart/being weakened.

So, for example the particular purposes were:
Austria - mainatining its empire
Germany - wanted to establish its power over the other countries
Russia - wanted to strengthen its own empire
UK - other political considerations (didn't want Germany to get too strong, wanted to support its allies to create counter-balance for Germany)

3. i would say one, but one is not prime nor composite.