- "I’ll put your brains in nice little jars, and then hunt the reapers down at my leisure."
- ―Desmond
Capturing and freezing the head of a servant is one of two ways to permanently defeat them, the other being the destruction or capture of their reaper.
Theory
Under normal circumstances, it is basically impossible to capture a servant whose reaper is far from the battlefield. This is because if the servant is killed their reaper can resurrect them at a different location. However, this only works if the servant is dead. If their brain is still mostly intact, it will be impossible for the reaper to do anything. This means that by destroying the rest of the body and keeping the head, it is possible to keep the servant trapped in place.
Value of Freezing
While most servants will not be particularly dangerous when reduced to a severed head, especially if the muscle is stripped from the skull to prevent any movement or power use, it is possible for a servant reduced to a head to report to their reaper and think in general, and if the muscle was stripped imperfectly the may be able to self-destruct. If the servant's head is frozen, this will prevent that from happening. The servant will still count as alive for the purposes of resurrection and presumably will still gradually grow in soul power, but they will not be able to think and their growth in the use of their servant power will presumably halt.
Escape
Though the servant and reaper themselves will be rendered unable to act once the servant is beheaded and frozen, the reaper will likely still be free to talk with their allies. If the reaper can arrange for other servants to recover or destroy the head of their captured servant, they will be able to resurrect them with no lasting harm. The same is true if the head is released or destroyed willingly by the capturing party, which makes captured heads a useful way to take hostages to negotiate with.