Spam uses a lot of bandwidth when you get emailed, thus causing charges to increase bandwidth. It also can cause sms charges when you get texts if you don't have unlimited texting.
Spam email will also cost you time, it takes a lot of time sorting through email to delete spam. That can cost a lot of money if your not working.
What is bad about Spam
Spam can spread malware that is used for fraud.
Ways to stop it.
Spam filters will help it. Also, do not reply. You can also use one time emails.