First, because I see many, many studies of pH optima in which a biological phenomenon is determined in a range of pH values, often controlled by different buffers. These experiments usually contain at least three variables: pH, buffer species and ionic strength. This stack will ease calculation of buffers at a fixed ionic strength and may eliminate one of these variables.
Second, because the stack may serve to remind users of the relationships between pH, ionic strength and temperature.
Third, because I think Hypercard is impressive. This is not a 'real' object-oriented stack, but was easy to write nonetheless.