Better reader experience. Let’s face it: plain text is BORING. Your blog may have the most compelling content on the internet, but give any reader a wall of text and he’ll run away faster than you can quote V. (The reference below is delightfully apropos.)
If you're blogging, you need images. These days, the Web is image-centric.
Use images to get attention for all your writing... and if you haven't done so already, make the most of Pinterest.