Calculating or retrieving a value only to then overwrite it or throw it away, could indicate a serious error in the code. Even if it's not an error, it is at best a waste of resources. Therefore all calculated values should be used.
== Noncompliant Code Example
----
void calculateRate(int a, int b)
{
int i;
i = a + b; // Noncompliant; calculation result not used before value is overwritten
i = doSomething(); // Noncompliant; retrieved value not used