Before you start your blog, figure out why you want to do it in the first place. Is it for personal or professional reasons? Some people start a blog to give potential employers a glimpse into what and who they are, a richer impression above and beyond that nerve-racking interview. A blog enables employers and/or customers to get to know you and like you before they work with you. Just remember when you decide to commit to blogging that, like everything else, what you put up on the Web will last forever somewhere in cyberspace.
Personal bloggers use blogging as an outlet, a way to express themselves. Blogs allow us to have online conversations with people who are interested in the same things we are. Whether it's creative writing, political writing, technical writing, there's a niche for it. It doesn’t matter if you’re trying to educate or illuminate or aggravate – just remember to stay true to your niche. Always keep your audience and your common theme in mind.
Why shouldn't you blog? You shouldn't blog just because you "think" you should blog. If you don't have the time and commitment - stop now. As Li Evans of KeyRelevance says, “If you don't have that passion to sit there and write blog posts and communicate with your audience, you shouldn't be blogging.” Your disinterest will show through in your blog. A stale blog or an abandoned blog does nothing for your reputation.