What is the role of POCl₃ in the reaction to form (NPCl2)3 (hexachlorophosphazene)?
If your goal was to prepare hexachlorophosphazene $\ce{(NPCl2)_3}$, 



then I'm not sure why $\ce{POCl3}$ was added.  The synthesis involving $\ce{POCl3}$ certainly works, but hexachlorophosphazene can be prepared more simply by heating phosphorous pentachloride and ammonium chloride in chlorobenzene.  Careful sublimation affords the desired product, pure and in high yield.  This route is more economical and less toxic since it avoids the use of $\ce{POCl3}$.

$\ce{POCl3}$ is typically added to the reaction when one is interested in preparing higher polymers of $\ce{(NPCl2)_{n}}$.  In this case, use of $\ce{POCl3}$ allows you to prepare and isolate $\ce{Cl3P=N-POCl2}~(1)$ in high yield and purity (ref. 1).  The purity is important in obtaining high molecular weight polymers of $\ce{(NPCl2)_{n}}$.  Condensation of $(1)$ to produce hexachlorophosphazene typically occurs when reaction temperatures are maintained well below $\pu{200^\circ C}$, while high molecular weight polymers are produced at temperatures over $\pu{200^\circ  C}$.



Reference:


Synthesis and Characterizations of Poly(organophosphazenes) By Roger De Jaeger, Mario Gleria. See page 26 here