The macrobes used a human head as an instrument of physical communication with their human pawns. We know that they can communicate with humans without the head, but that tends to scare the daylights out of their pawns, so the head is a useful charade. With the original head destroyed or out of commission, the macros needed another--mostly for their own convenience in communicating, and centering their pawns' loyalty around something vaguely human.
But I think Lewis may also have been making a larger, more metaphorical point here. Evil spirits have always used humans as pawns. In fact they can't accomplish much without them. The macrobes need the cooperation of humans to maximize their nefarious results. They also care nothing for "the head" or anyone else, as soon as that person has served his purpose and is no longer of value. The macrobes truly use people in the worst sense of the word. This is in sharp contrast with the good guys. Ransom and his "spirits" also, in a sense, use people. But those people are not discarded and abandoned to destruction as soon they have served their purpose. The opposite occurs: those who have been "used" are elevated, empowered, preserved, and blessed.
To make an even larger point, there are numerous remarkable similarities and contrasts between Belbury and St. Anne's.
Consider first the similarities:
- They both have people and those people do work.
- They both have headquarters at a physical location.
- They both have a "head" that is clearly in charge.
- They are both at odds with the "other side". They are enemies, literally and figuratively.
- They are both secretive, but the reasons for the secrecy are quite different for each side.
- Both societies have a head that is governed by spirits from another realm.
Now consider their differences:
- St. Anne's consists of flawed but nice people; Belbury consists of very nasty NICE people!
- Belbury obtains and controls its followers using greed, threats, fear, seduction, and promises of power. Ransom obtains and influences his followers using love, respect, and protection.
- Belbury desires power and dominion. St. Anne's desires life and righteousness.
My point regarding the similarities and contrasts is that Belbury turns on itself and cannibalizes itself -- symbolized by the demand for another head and the murders that follow. While St. Anne's comes out better, stronger, and safe at the end!