It's not needed, but it can be useful.
The important bit in defining the use of og:title is "as it should appear within the graph." In general, you want that to be the title of your blog post(or whatever). If your site's HTML titles are just that, then there's no real difference here.
On the other hand, many sites produce title tags that look like some combination of:
Blog Post Title | PostCategory | Site Name | Site Blurb Here
…in which case all of that gets sent through as the Open Graph data.
This would be a case where you might want to set og:title to the post title, so that when something gets shared to Facebook, it comes through without all the clutter.
There are several potential cases, eg. on some sites a post's headline is entirely different from what ends up in the title tag, which will often be a more "SEO-friendly"/linkbait-y version. Here again you might want the og:title to be the true headline.
