There's only so much traffic you can get from the homepage or About Us page of your company's website. Of course, those pages are critical for leads who are already interested in your products -- but they often won't attract traffic from top-of-the-funnel. That's where your blog comes into play.
Your blog can be a general resource to help your website visitors even before those visitors are ready to purchase from you. For instance, let's say you sell products for e-commerce stores. You might attract some e-commerce owners who are already searching online for your products, but in most cases, the e-commerce owner isn't going to be ready to buy right out of the gate.
Alternatively, if you begin blogging about tips to help the retail owner who is just starting out -- like "How to start a retail website", or "Benefits of e-commerce vs. physical store" -- you'll slowly attract an audience who enjoys your content and finds it useful. Then, when those site visitors' e-commerce stores begin growing (thanks, in part, to your blog), they'll already know about your brand and already trust it as a helpful source. That's when they'll check out your product pages.