what is the use of state rank and category rank???

Admission to all colleges that come under your state and provides admission on the basis of jee main score will consider your state rank, while all other colleges outside your state will consider all india rank.
State rank, it means you rank as compared to all other students that belong to your state and all india rank it means your rank compared to all other students who appeared for jee main,
In every college some seats are reserved for state, tht's why you will get two ranks, State rank and All india rank