Bitly has made it even easier to see and track your link clicks. So why would you want to go through the extra steps to shorten a link, especially if you already have analytics set up on your sites?
Many times, the link you are posting might be to someone else's website. It could be a link to a payment page on Paypal. It could be linking to your friend's new blog.
You'll want to be able to see if any of your followers clicked on this link so you know if the content was interesting to them or not. This helps you determine what you should be posting more or less of. In the case where it might be sending them to a payment page, you'll want to see if people clicked through and stopped short of buying. These are just a few examples of why it's nice to have more control over the analytics of your content links.
Also it's nice to have a small shortened link instead of a really long cumbersome one. That reason alone sometimes warrants the few extra seconds it takes to shorten a link.
Go check out bitly.com and start playing around!