There isn't one answer. Just as there isn't for having a social media presence. The main reason to start a blog should be having something interesting to say,

There isn't one answer. Just as there isn't for having a social media presence. The main reason to start a blog should be having something interesting to say, on a regular basis; something that appeals to your market. A message that either converts into sales or (if a service) converts into an impression you know what you're talking about. The SEO benefit isn't automatic. Just having more pages about something on your website doesn't turn into positive SERP. It only helps if people in your industry share it around, discuss it. And that not only depends on the quality of your message but also how "bloggy" your audience is. I have clients whose market is furiously corporate and they insist on blogging. It's just such a dry topic, there's nothing but a "we're a real company" benefit, and even that isn't obvious. And that leads to the biggest issue here. It's often not easy to see if you're wasting your time. It's easy to throw terms like "sunk cost fallacy" around but it really applies here. Plus there is auxiliary value from blogging, some people enjoy writing. But if your time is finite and not worthless, focus on finding an audience that directly feeds into your bottom line.