As an independent author, you obviously hope each new reader will appreciate your writing. But your fans are also a crucial component in your book-marketing efforts, helping you spread news about your career through word-of-mouth recommendations, social media sharing, and more.
In other words, without turning customers into readers, and readers into fans — your path to success will become impassable. So, it’s important to get as MANY people as possible to become customers. Here’s how that happens, in reverse order. 
Before someone buys your book, they’ll probably read your book description, blurbs, bio, and maybe even skim a few pages from chapter one.
Before they can do any of that reading, they’ll have to pick the book off a shelf, or perhaps click to the book’s page on Amazon, B&N, etc.
And before they are compelled to click a link or pick up a book, they’ll have to be attracted to your book’s cover (or thumbnail image).
Then if we rewind far enough, there’s the marketing/promotion work that brought this potential customer to the place where they could even view your book cover (on an end-cap or shelf, or on a website, blog, eBook retail store, etc.)
But let’s go back to step 3: your book cover HAS to be compelling. At this point in the game, when you’re trying to attract as many customers as possible, your book cover is THE most important part of your book. At this point in the game, the book cover is not just the first thing someone sees; it’s the ONLY thing someone sees — and they must decide based on the book cover alone whether they’re going to take some additional action to investigate what’s inside your book.
A book cover is an invitation — a way of seducing the reader. It beckons, inviting them to enter the world of your book and dance with your characters for awhile. It makes a promise about what kind of music they’ll be dancing to.
Your cover should convey the tone and genre of your story, be eye-catching and, most importantly, look like it’s been professionally done.