I believe very strongly that as an author, you must bring a strong knowledge of who your audience is to the table.
I believe that authors who build writing careers do it by knowing their audience and growing their audience. I have watched authors take a "If I build it, they will come" approach. I have never yet seen an author succeed with that. One day I will, but it'll be the exception that proves the rule.
Publishers do not deliver readers, with a very few exceptions. They deliver access to retailers, sure... the big ones. They deliver some marketing effort, especially if you have a recognizable last name. They deliver cover design, editing, and production with varying degrees of input from the author. But, they don't deliver readers. There's no magic bullet that allows them to ensure that you'll sell enough books for the effort to have been worth their while.
(Total aside: I think this has to change. I think publishers who successfully navigate the sea of change that is book publishing will do it by being champions of a certain type of story and connecting with the audience for those stories. Then, they'll continue to deliver great books to that audience. If you want to be a publishing entrepreneur, do that.)
What does all of that have to do with design and branding?
Your branding (both the words you write and the design of everything you put your name on, from your website to your facebook page to your book cover to your bookmarks) has to suit your audience. I see mis-branded books all the time. I see misbranded facebook efforts even more frequently. The thing is, people respond to certain cues. Mystery readers see a romance cover and check out. If your epic fantasy looks like a rivets and bolts space adventure, your audience will disconnect. If your facebook branding makes you look like an edgy suspense writer with lots of jewelry, your audience will be disappointed to find out that you write Amish fiction.
What that all means is that, to some degree, you have to be aware of the message your brand is sending at all times. You own your brand.
That sounds exhausting, doesn't it? That's why the most successful writers are the ones who write the things they love. Then, they're able to just be who they are while still connecting to their audience with a good, well-designed brand. When the core of your brand matches up to the core of your person, you can get rid of the exhaustion and anxiety of marketing and just be yourself.