Igneous rocks are known as primary rocks for two reasons:-
1)Igneous rocks are formed due to cooling down of lava and are directly and simply produced in nature
2)These rocks give birth to metamorphic and sedimentary rocks.As:-
the igneous rocks break down into small sediments,which combine under high pressure to form the sedimentary rocks .
the igneous and sedimentary rocks under high pressure and temperature,form metamorphic rocks.
Hence ,igneous rocks are known as primary rocks.