Here is why:

• there are also hundreds of blogs out there
• you're not an expert
• you are showing the world how stupid you are
• people noticizing your blog will trash it

On the other hand,

• it is notes to your future self
• release ideas that you have in your head
• learn to write and express ideas
• add a personal view
• writing about a subject can lead to being someone's hero some day.
• it's cool
• you can write whatever you want