You benefit from have a subscriber form on your site because:
It gives your visitors the option to follow you more closely.
It shows they are interested in YOUR CONTENT and what you're doing with your product or service.
By the above factors, it also increases the chance of "word of mouth" advertising, Facebook follows, etc.
As your list builds over time, it provides you with the chance to reach out to actual people who are interested, which would be deemed as real traffic. Every time you update your site, you can take advantage of your OWN email list built from scratch.
I do, however have to warn you about having a subscriber form on your site.
BEWARE OF SPAM!!
It will find your form and try to spam your site to death.
How can you avoid such annoying pests?
There are a couple things....
Ensure your Akismet plug-in is activated, as it will help deter such varmints...
Set your comments up to be moderated by you, so you can look at them yourself and determine if they are actually spam or not.
Captcha Code plug-ins are useful as well to ward off the pesky critters.
I hope this answers your question regarding subscriber forms
