By factoring expressions in your programs, you can potentially eliminate a lot of unnecessary computation.
For example, the first block of code below is more efficient than the second block of code:
MOVE ZERO TO TOTAL PERFORM VARYING I FROM 1 BY 1 UNTIL I = 10 COMPUTE TOTAL = TOTAL + ITEM(I) END-PERFORM COMPUTE TOTAL = TOTAL * DISCOUNT
MOVE ZERO TO TOTAL PERFORM VARYING I FROM 1 BY 1 UNTIL I = 10 COMPUTE TOTAL = TOTAL + ITEM(I) * DISCOUNT END-PERFORM
The optimizer does not factor expressions.