An estimated 90% of all emails are spam. Deleting them is a big time-waster. Spammers are getting ever more sophisticated and producing very plausible-looking messages which can fool even the wary.
Crucially many SPAM emails contain viruses or trojans which can really destroy your life. Basically this BBC page is a nice little list of reminders.
As a footnote some of the big email providers such as Google have excellent SPAM blockers built into them. However don't forget to check the Junk folder every so often!