Cost, Time, Content
The more time people spend reading and deleting spam emails, the less time they spend actually working and making money. Therefore, spam costs people money.
Someone who gets a lot of spam emails waste more time on their email than people who don't. Spam takes up a lot of time.
In some spam emails, there are links to other websites that can end up being frauds or viruses. So it is bad to ever click on a link in a spam email, or reply to the spammer.