زَيْدٌ مُنْطَلِقٌ
Zayd is setting out / is on his way.
كَانَ زَيْدٌ مُنْطَلِقًا
Zayd was setting out.
The first construction attributes departure to Zayd; adding «كان» in the second places that attribution in a past temporal frame. «Kāna» is therefore not an empty addition: it modifies the temporal interpretation of the predication.
A speaker wishing to describe a past state may need the second form. A speaker intending a present or unqualified characterization should not substitute it without noticing the semantic shift.