If you want to make money from blogging, or work with brands and PRs, having a media kit is essential. I have heard of bloggers who don't have them, and are successful, but having all your information contained in one place that is easy to share with people is a much better idea than trying to remember facts and figures, I reckon.
What is a media kit?
Put simply, a media kit is a resume for your blog.
What do I put in it?
A media kit should contain a brief bio about you, and your blog; your web address, and how to contact you; your rates, if you charge for sponsored posts and advertising; all your social media channels, and the number of followers; your google analytics sessions, users and page views (I've heard of people being asked for screen shots of these stats, so I just include it)
What should I charge?
You'll get mixed answers to this question from bloggers. Ultimately, you need to charge a rate that you're comfortable putting out there. I heard in a podcast recently that you should set your rates high enough that people say no thanks sometimes (Elise Gets Crafty. Afraid I can't remember which episode.)
Do you have a media kit?
How much should I charge for a blog post? | Secret Bloggers Business
Sharing what I said at ProBlogger | The Remarkables