Email marketing and newsletters allow you to engage with your customers on a regular basis to keep them invested in your company without being pushy. That is, if you do it right.
Customers sign up to receive emails from you, then you can regularly, but not too often, reach out to them with updates, useful tips, and news. It serves a similar purpose as an ad, in that it is drawing someone's attention to you, and enticing them, but it differs because these people are already interested, they already know your business, you're just maintaining a relationship with them, keeping your business in their minds.
It is crucial that the relationship you're building is a positive one.
If you send spammy, pushy, or uninteresting emails, customers will stop opening your emails. The emails will start going to their junk/spam folder and not be seen. Or the customer may cancel their subscription to your emails. Even worse, they may make customers distrust your business.
That's a lot of negative. Let's focus on what makes email marketing and newsletters work.

First, you have to provide useful content. The emails can give readers tips, inform them of new products, or even just entertain, as long as customers benefit from them and they fit your brand.
Email marketing will often include a call to action or a plug while newsletters usually don't.
You want to be mindful of how often you're sending emails. Even great emails can get to be too much if people receive them too often.
I have experience running email marketing campaigns, and I focused on providing valuable content and selling a minimal amount. One business owner kept getting compliments on my emails!
Contact me to write your emails, plan an email strategy, and maybe even to maintain the email campaign to ensure people are responding to them the way you want them to.