1. You need to keep practicing your storytelling skills with some social accountability built in.
2. You just love to write but don’t have anywhere else to put your words (yet).
3. You’ve thought through the idea (at least a little bit) and truly believe you can make it work and stick with it long-term.
4. You eventually want to write a book on Topic X, and blogging about it might help you reach that goal someday.
5. You’re trying to decide on a “niche” your writing fits into. A blog focused on one niche may be worth trying.
6. You have thoughts/opinions/ideas and want to share them all in one place.
7. You’re a credible authority on a topic and want to share information with a curious audience.
8. It can be a good way to connect with other writers/future readers, if you’re smart and strategic about it.
9. You want to share your experiences with an audience — your story might help a stranger, or a friend.
10. Posting on a blog consistently might help you keep up with other writing projects, too. Sometimes, the busier, the better. Sometimes.
11. It’s something you want to do — you’re even a little excited about it!
12. You know it might not get you a book deal or a job anytime soon. But it’s, at least, a place to start.
Meg is the creator of Novelty Revisions, dedicated to helping writers put their ideas into words. She is a staff writer with The Cheat Sheet, a freelance editor and writer, and a 10-time NaNoWriMo winner. Follow Meg on Twitter for tweets about writing, food and nerdy things.
Meg Dowell Writes 
I would add that it’s a great “proof of concept” small scale simulation of a writing profession. It creates a sense of “writing to deliver to a deadline”. It also gives you a “purpose” for writing, a hypothetical audience that you feel obligated to write for, and write well. And I think there are many lessons that can only be learned by writing numerous independent pieces, each written to stand alone, and maybe be part of something larger. One of the biggest lessons, in my opinion, is the idea that it’s not about perfecting the one piece; rather it’s about learning and improving by writing many, becoming someone who can write a quality piece in a timely fashion, even if you’re tired, or just not feeling 100%, and then going back through the assortment of completed projects and picking out which one best fits the audience you’re submitting to today.
I completely agree with all of this. I’ve honestly learned much more about writing through my blog than I ever learned getting my degrees (though the education as a whole was still worth it). Real-world experience is more valuable than many realize.