Each blog post needs a featured image, and they all need excerpts. If you forget, you'll know immediately by looking at your Blog page because, well, it will look really funny — there will be no text or image to accompany a post, so all you'll see is a date.
One thing you'll notice is that the title displays on the blog post itself, but it doesn't display on the archives. We use excerpts to try and lead people in. This shouldn't have a negative effect on your SEO, so don't worry! This is exclusively for the sake of presentation.