Here is why you need a landing page:
Convert Into A Potential Reader
Your blog's RSS Subscriber count is an important metric of your blog's success. A good landing page can convert lurkers into readers. Just putting a small email subscribe box can do wonders.
Visitors May Feel Deserted
However good your blog's design is your visitor is new to it. The visitor may find your blog strange. So, there are chances that he might leave your blog, thus increasing to your bounce rate.
You really don't want this to happen, so you must explain things.
Increase Visit Length
Putting links to popular posts on your blog (which can be done with a simple plugin) on your landing page can help old posts to get noticed. It will make your visitors hang around.
Have you put a landing page on your blog? What all do you have on it? Have any tips to share?