Many readers will not read everything you are writing. They will skim through everything you present them and will decide which of the things interest them and which do not. The most important way to get readers to decide whether they want to read your blog post is the title. In general it should be short and tell the reader the main point you want to bring across or the main thing you are talking about.
Once your readers found a title they find interesting they need to decide whether they actually want to read it or not. If they have to click and wait for the whole blog post to load a bit they will be more reluctant to "test" every interesting sounding title. A second or two can decide the fate of a website, especially if they have to click, read a bit and then go back to the main navigation. That's why little teasers are very popular. You can place a couple sentences under the title of your blog post on the page where you are displaying all of your content and thereby make it easier for readers to see more details about what they are going to read. This helps them to make their decision because now they can get a better idea of what the blog post will be about without having to open the whole post.
Teasers are also great if you have the possibility to link to your blog post from a different website. Another website won't allow you a lot of room to advertise your post and you wouldn't want to copy-paste your whole post somewhere else - you want people to get to your blog after all. Having a teaser at hands makes it easy to copy-paste that together with a link to get people to your blog.
A teaser also tells you readers a lot about your writing style, which is very important. You wouldn't want to read something that you personally find incomprehensible, especially for a longer amount of time. And depending on the author and the topic blog posts can get quite long.
It still needs to fit the UI. As you can see on StackExchange for example only the title is displayed and it still works just fine. In this case the title is even more important and the advantage is that you can display lots of content on a single page without the user having to scroll. But if you are writing a blog alone you will likely have a lot less content than a site on the StackExchange network. Look at how for example Medium.com does this. I have written some blog posts for the Universe Factory there, the unofficial blog from our sister site WorldBuilding.SE, and on Medium you always have a picture and more room between posts to give authors the chance to post their little teasers. We also use the teaser to announce the blog post on the corresponding meta post, though that is often just an excerpt from somewhere at the beginning of the individual blog post.
All in all: there are some things to consider, like how much space you have, how much time you want to invest into creating a teaser and whether it's enough to simply copy-paste a bit or write a completely separate teaser, but in general it's a good idea to give your readers a little "taste" of what they are going to read before presenting your whole blog post.