It is highly recommended that digital marketers should have some skills in graphic design and even HTML or CSS coding. Why is this, you ask? Because depending on which platform you choose to advertise on, some of these skills may be required. For example, on Facebook Ads, the text must only represent 20% of the image space and, the rest is the image itself. Facebook and Instagram are very much image-driven, so copywriting is less required, which is why having some basic graphic design skills can be useful. And, let's not forget that Google Display are also visual-driven types of ads most used for retargeting. As for HTML and CSS coding, digital marketers may be required to install snippet code on websites or landing pages to collect data, so some basic understanding of coding is always helpful.
That is why I purchased the official Adobe training workbooks for Photoshop (ed. 2019), Indesign (ed. 2014), Illustrator (ed. 2018), and Dreamweaver (ed. 2015). Most of the books I bought are second-hand since the more recent ones are more expensive. I don't necessarily recommend reading the books in their entirety unless you want to be an actual graphic designer, but only the sections that are relevant to your activity. In the case of Dreamweaver, I only read the sections related to understanding HTML and CSS coding and, for Illustrator, I just read how to draw some basic icons. Finally, the great thing about having these physical books is that I can always go back if I need some information on their usage.