Answer:
To prevent help to the Kathmandu valley.
Step-by-step explanation:
It was necessary to conquer Makwanpur in the unification Campaign of Prithvi Narayan Shah because Makwanpur helps by exporting goods to the Kathmandu valley which was against the policy of Gorkha. Makwanpur had control over the southern route which goes to the Kathmandu valley so they provide help to the valley so for that reason Prithivi Narayan Shah thought to conquer Makwanpur.