Hempel, in his 1987 article "Residual Finiteness for 3-Manifolds", shows that if 𝑀 is a compact Haken 3-manifold, then 𝜋1(𝑀) is residually finite. In the proof he starts by reducing the case to '𝑀 is closed and irreducible'. Why can he so easily do that?

Thanks in advance!