مَوَاضِعُ الْوَصْلِ بِالْوَاوِ: يَجِبُ الْوَصْلُ فِي مَوْضِعَيْنِ
Two principal contexts call for joining sentences with wāw.
First: The sentences are alike in being declarative or nondeclarative, share a fully suitable relation, and face no impediment to coordination.
Second: Omitting wāw would lead listeners to a meaning opposite to what the speaker intends.
The first coordinates legitimately related utterances. The second protects the intended sense from being misconstrued. They are distinct reasons, not merely grammatical preferences.