Scenario: You're given 25 pcaps from a certain time frame and asked to determine if there's any emails in them to or from email@example.com. You need a way to determine what pcaps might have this data in it. You can use rail grepping to determine this.
rail grepping is named after the parameters you'll use with grep to do your search.
r - search sub-directories recursively
a - treat binary files like text and search the ASCII, human readable portions of the file for the string
i - case insensitive
l - only show the files that match instead of the matches themselves.
So our command would look like this:
grep -rail 'firstname.lastname@example.org'
grep will rip through the pcaps looking for your email address and return the name of each file it finds a match in.