One type of digital marketing - Email Marketing

 https://blog.hubspot.com/marketing/what-is-digital-marketing#role-of-digital-marketing

Email marketing is a type of digital marketing that make consumers aware of discount information and new products. When people went shopping in the shopping mall, different brands will encourage customers to sign up an account by using email address. They will also provide new user discount to lead customers sign up and leave email address. Since consumers won’t check each brand’s official website or go to stores every day, email marketing helps to build a solid relationship between consumers and companies.

In Gmail, there is a sort named "promotion". All advertising emails are in this sort. 

So I am curious about ROI on email marketing. Since those emails won't come to primary inbox, I won't check them at all.

Here are screenshot of my promotions inbox. Those advertising emails remain unread since I won't check them and I received too many emails from those brands.