The Tool
I'm finishing this blog post up at about 6am. Once I finish, I might not want to immediately share it on Twitter because most people aren't on twitter yet. Buffer makes it easy to queue up items to share on social media at the best times.
It also helps so you don't flood your followers with a bunch of content at once and then have silence for the rest of the day. If you find 5 articles you want to share, buffer will space them out for you over the day and at optimal times.
Another great feature is to schedule items to repeat posting. Each of your readers looks at Twitter, Facebook, and LinkedIn at different times in the day. If you're only posting at 2am when you finish the blog post, a lot of your potential audience won't see the post.
Finally, Buffer offers analytics on how many potential viewers there are for a post, how many people clicked on a linked, liked a link, etc. You can use that information to determine what information bring the most traffic.
Buffer is free for the first 10 items in your queue. If you need more items, more social media accounts, and RSS feed options, there is an Awesome version for $10/month.
For more Information on Buffer
Alternatives
There are a lot of alternatives out there. Hootsuite is one of the more popular. It's main focus is managing social media in general. Buffer is specifically for scheduling and analyzing posts.
Edgar is the rich uncle compared to Buffer. For $49/month, Edgar provides better scheduling, improved analytics, and an endless queue of posts based on categories.