You might want to check this article, below is a direct quote from the page which sums up the power of eval.
The JavaScript EVAL command can be
very powerful when using dynamic
content in Web-based applications.
EVAL can reduce code and eases
interaction with data that is unknown
at load time, but there are drawbacks
to take into account.
The real power of this command is its
ability to work with data that is not
known at load time—for example,
user-entered data. As I have
mentioned, EVAL takes any string that
you provide and executes it returning
a result, if one is applicable. You
can use this to execute JavaScript
where what you are being asked to
execute is not known at load time. For
example if we have a calculation
script that takes an equation provided
by the user and returns a result, you
can use EVAL to perform the
calculation.
