When you buy a domain you are asked to provide contact information.  This information will be publically available using a WHOIS query.  This means that the email you provide may well be hit by spammers or other marketeers.  You can pay to make the information private or you can just provide information that doesn't cause you grief to have revealed.  Beware however because if someone asks to do something to your domain, the email you provide will be the only notification you get and there will be problems if you don't check this account.