Final answer:
The primary reason the nurse must frequently assess tube placement is to prevent aspiration of the feedings.
Step-by-step explanation:
The primary reason the nurse must frequently assess tube placement is to prevent aspiration of the feedings. When a nasogastric small-bore feeding tube is inserted, there is a risk that the tube may be positioned incorrectly and the feedings may enter the airway instead of the stomach. This can lead to serious complications like aspiration pneumonia.