Once you have registered a domain, the email that you used when filling out the WHOIS information will be visible to anyone who looks for information about to purchase a private domain when checking out. Spammers have been known to use this public information attached to domains to send unsolicited emails. This could result in an increase in the number of spammy emails you receive to that email address.
If you wish to protect your email address from potential spammers, you have the option to purchase a private domain when checking out.