Would you give your home address to a perfect stranger? Probably not, so why are you ok with giving out your email address? The unfortunate answer is the internet forces you to. Every account registration online asks for an email confirmation in an effort to prevent fraudulent accounts and bots. Even if you are using Google or Facebook to login you are likely still sharing your email address.
While you should be able to unsubscribe from unsolicited email, you are rarely able to unsubscribe yourself from the affiliates. This puts the burden on you to receive the unsolicited affiliate emails and then go hunt down the unsubscribe process. Also keep in mind that unsubscribing only marks your information as unsubscribed. The company is likely still storing and sharing your information.
2017 was a landmark year for data breach disclosures with literally billions of users compromised most notably Equifax. The data lost may have provided hackers with a buffet of data about you including information about your passwords. Your seemingly innocuous email address is the key to hackers combining personal information about you from multiple sources which can be used to gain access to your online accounts.