The free version allows you to set up one Twitter account and one Facebook account. (If you want to connect more of your social media accounts and have more functionality, you can upgrade.) They've got handy goodies like browser extensions, mobile apps and buttons for your blog.
What makes it handy?
It's helpful if you like to post a lot but don't want to do it all at once and flood your stream or feed.
It's also nice to be able to set your schedule to post at the times you are likely to get the most engagement.
Also, I love being able to surf the net and add things to my Buffer super quick using the bookmarklet.
Disclosure: There are affiliate links in this post.