Content that offers value to the customers, for example, a discount or sale would elicit an instant response. But, emails are not just meant to inform customers about offers and discounts.
There is a journey that each brand has to travel vis-a-vis each customer to drive him/her to reach to the end of the sales funnel and ultimately make a purchase.
Through emails, you connect with the audience, build interest in your brand, build trust and then get down to talking about the sale. When you directly talk about the sale, it might generate a positive response once or twice but, to convert that customer into a loyal one, you need to go through this journey.