Links are shortened for the following reasons:
- It helps you stay within Twitter’s character limit. Links can be long, and shortening them saves you some much needed characters.
- Twitter’s link service measures how many times links are clicked. It helps them to compare tweets to similar ones and see which perform better.
- It let’s Twitter screens for links to malicious sites that seek to harm users. Using their link shortener, Twitter is able to identify threats and warn users about it before they go through the link.
You cannot opt out of using Twitter’s link shortener. But that shouldn’t be much concern, since it only seeks to help you. You can still use your own link shortener, with the ability to track metrics, if you are so inclined.