Consider the following problem:
Example 8: If a score of 90 and above is an A (4.0), a score between 80 and 89 is
a B (3.0), and a score between 70 and 79 is a C (2.0). What grade will
Joan, Jane, Jim, James and Jill get if there scores are
87, 95, 72, 16 and 83, respectively?
You can not use:

grade = 4.0

because all students did not get an A. You must be able to
select certain students (rows) based on their scores. The
"wherever" statement allows this.
See Example 8