What are the purposes of AddRef, Release and QueryInterface functions ?

Add Ref : Increment the counter (Interlocked Increment) when the component initialized.

Release : Decrement the counter when the component destroyed.

Addref and release ref is done to control the de-initlaization of COM dll
com dll interface can be used in many places .But Co-Unitialize should be done only when all the clients are done and released the interface. this is achieved by tracking the interace usage count.