Reason 1: Direct Mail can Support Email Marketing
The first reason that doing both direct mail and email is that one can, and should, support the other. In the example of the postcard I got, I took note of the email option and acted on it. Instead of making me choose email over direct mail, suppose the trend of acting online from the direct mail continued. Future postcards could draw my attention to email only offers, online store offers, company announcements, releases, newsletters, etc.
The point is that part of marketing is how many times you meaningfully touch your target audience. Doing email and direct mail allow for varied media that has the potential to resonate better with the prospect. So if you have a few avenues to approach them, you increase your odds of them taking the next step.