Powerful NPCs and monsters are needed to establish a power scale in the world.
An NPC in a position of power like Halsin as an archdruid should be completely out of your league in the beginning. Then by the end of the game you're more or less his equal and that reflects your growth during your journey.
Or they can be narrative tools like Gorion in BG1. A powerful old wizard but he is killed in front of your eyes. This tells you how powerful your nemesis is and puts the fear in you.
If Halsin would be lowered to companion status around level 4, an archdruid would mean nothing in the setting where a PC is always the "top dog" regardless of level. No room for growth if you're always as powerful as the "most powerful" NPCs.
