If you run a DIY baking blog (your niche), companies that sell cookware are likely going to be one of your top supporters when it comes to making money blogging.
You'll have a solid idea of who your target audience is (at-home bakers) and exactly what they're hoping to accomplish (looking to improve their skills, get easy baking recipes, learn more about cooking in general). On the flip side, this also means that you can confidently communicate with sponsors about who's consuming your blog content—and more importantly—why they're likely to buy the advertiser's product.
Listen to my interview with famous food blogger, Gaby Dalkin of What's Gaby Cooking, to hear more about how she went from writing on the side, and learned how to make money blogging on a massive scale with advertisers, book deals, her own line of salsa products at Williams Sonoma and more. See my guide on starting a food blog for more if that's the niche you'll be blogging about.
After hearing this example, now imagine what it'd be like if you didn't have a niche to blog about. Maybe you write about caring for plants, but you also like to write about psychology, with a bit of hiking sprinkled in. Not only is your audience all over the place, but they're probably a little confused too. And that in turn will confuse your advertisers.
For that reason, I've chosen to launch my own hiking blog, Hike with Ryan. I'm doing this as a new side project with content like my guide to the best hiking trails in Yosemite National Park, a roundup of the top hiking boots around, my picks for the best hiking gift ideas, when to visit Yosemite and more—all as not to confuse potential advertisers and sponsors on my main site here.
Gardening companies probably won't want to advertise where someone is reading instead about psychology-backed parenting techniques.
I know this might sound like an over-simplified example, but finding and sticking to a niche will help you stand out amongst the crowd of unfocused content, and will lay a solid foundation for your blog.