People who subscribe to your blog will get notified when you have a new post there. As an author, that's not what you want. You want them to subscribe to a specific newsletter, which you can use to send them news of new releases and/or book news or whatever you deem appropriate. (Some authors send regular, chatty newsletters about all aspects of their writing, with cover reveals, book reviews, notice of sales by them or other authors in the genre, etc, while some authors only want to send email to their subscribers when they have a new release. There are pros and cons to both approaches.)
If they subscribe to your blog and get notified every time you post they may quickly get tired of you (depending on how often you post and what you post about) and/or ignore your posts. For instance, I don't just post about my books on my blog. There are funny stories about little things, thoughts about writing, photos from holidays, etc. Someone who only wants to know when I have a new book available isn't going to want to get constant notifications about stuff like that.
Also, I'm not sure--but someone else will probably come along who knows--whether a blog subscription would satisfy the CAN-SPAM laws, or how well Wordpress would handle the traffic if you ended up with thousands of subscribers (as we all hope we will!).