The constant parts of expressions are evaluated at compile-time so that PARAMETER statements can be used in many cases to make programs more readable without increasing execution time. For example, consider the following:
PARAMETER
(PI=3.14159)
. . .
CALL FRED(PI/2.0)
The expression PI/2.0 is constant and is therefore evaluated at compile-time and nothing would be gained by replacing the expression with its calculated value.