It is built into every person, I believe, to be heard, to voice their opinions. I think this is especially true about those who are shy or don't have the courage to give their opinions. When you can't voice your opinions aloud, and even if you can, a blog can be your voice to the world. It can be a semi-anonymous way for you to voice your feelings to the world, even when you can't summon the courage to say it with your mouth. Blogs and online public journals allow more public opinion to be heard than would normally be so. They give people a free outlet to yell, scream, praise, or just chat about whats on their mind. For many, this is their only outlet.
I believe that voicing your opinion in a blog can be very therapeutic as well. Allowing those strong feelings out not only helps you release them, but also just makes you feel good and makes you feel like you are doing something worth while.