One is the start-end pair of regular expressions. Actions with the END pattern will happen after all lines are processed. There are several choices. It is also useful for balancing one's checkbook, if the amount of the check is kept in one column. Which one depends on how they are used, of course.
Variables are also useful for keeping intermediate values.
It doesn't hurt to put it in, though. Integer variables can be used to refer to fields. The special variable NR tells you which record this is. It is incremented each time a new record is read in. Of course, there are a myriad of other ways to put line numbers on a file using the various UNIX utilities.