The point is to see how much space is used by the stack over a long period of time.
The stack is highest part of memory.
(all other variables are below the stack)
At the moment I set SP to $BF giving me 64 byte of stack room. The upper 16 bytes are never used, so
I could set SP to $CF giving 48 byte stack.
Ofcourse if you don't need much RAM space it's OK to set SP just above the highest locates.
But I need every byte I can get.