Is there any rule that passengers who have completed their journey should leave the airport?

I am a first time flier so please help clear my doubt -- After my flight arrives to Beijing airport do I have to leave the airport , or can I wait there , do I get to sit down somewhere with my luggage inside the airport (may for 4-5hrs) ? My point is - is there any rule that passengers who have completed their journey should leave the airport (because my friend may take some time to receive me from airport).