Why ? Because a huge population on the web still don't know what to do with that orange XML icon on your blog and even if they do, they find a traditional email newsletter more convenient than managing a feedreader.
And this is true even for techie blogs where a good percentage of users already have an idea about RSS.
Take for example the Google Adsense blog - they have some 1,500 subscribers on Bloglines but more than 10,000 readers have chosen to receive new content via email newsletters (powered by Google Groups).
Now these numbers from Google just underline the fact that email subscriptions can sometimes be more important than RSS feeds.
And when we look at our feed stats, the email subscribers constitute a healthy 30% of our entire subscriber base who get an email every weekday at 2 PM IST containing a summary of new content posted on Digital Inspiration.
You are probably missing lot of potential subscribers if that email subscription form is missing on your blog site.
Related: Jakob Nielsen - "It's really dangerous to design for a technical elite. We have to design for a broad majority of users."