I have an idea of what this button does, but I cannot find any specific information in the documentation.
What information does this button add to the profiling data? How can I see this extra information?
Consulting the Unity documentation on the CPU Profiling tools:
The recorded memory allocation samples appear in the Profiler window in bright red.
...If the callstack was recorded for the selected allocation sample, the associated callstack symbols are resolved and displayed as well:
So: when you have "Allocation Callstacks" enabled, you can click on the red boxes representing managed memory allocations to see what code caused memory to be allocated, and where it was called from.