Paul clarifies how believers are to relate to unbelievers: we are not to participate in the deeds of darkness. Then, Paul builds on the light metaphor to show how believers can be used by God in conversion.
Paul once again returns to the issue of immorality to ensure that none of it seeps into the community of saints. And the first place where this must be checked is our speech. Our lives are to be holy, and so our speech must be holy.