Read the excerpt from The Code Book.

More than a hundred million e-mails are sent around the world each day, and they are all vulnerable to interception. Digital technology has aided communication, but it has also given rise to the possibility of those communications being monitored. According to Zimmermann, cryptographers have a duty to encourage the use of encryption and thereby protect the privacy of the individual:

A future government could inherit a technology infrastructure that's optimized for surveillance, where they can watch the movements of their political opposition, every financial transaction, every communication, every bit of e-mail, every phone call. Everything could be filtered and scanned and automatically recognized by voice recognition technology and transcribed. It's time for cryptography to step out of the shadows of spies and the military, and step into the sunshine and be embraced by the rest of us.

In the excerpt, the purpose of the statistic "more than a hundred million e-mails” is to show that

digital information is widespread and available to monitoring.
most e-mails that are sent are not viewed by the government.
encryption has been used by the military but not ordinary citizens.
e-mail is a good way for people to communicate nowadays.