How so? By shortening the link, you're hiding where the link goes to. bit.ly/12345 could go to amazon.com or big-scam-with-a-virus.com, and until you click on it you'd never know.
With bit.ly specifically, add a "+" at the end of the url to see what it points to. It also shows you some stats like creation date and number of clicks over time.
It's useful to know the domain of the link before you click because some people might not want to navigate to unknown sites at work, or at least don't want to navigate to certain sites at work (Facebook, Instagram, YouTube, pornhub, etc, etc.)
You don't need a link shortening service for that. The website and API can just start changing the URL it includes in the tweet if it determines the original URL is a scam.
They can redirect you anywhere. They can also rewrite anything in the URL, like add affiliate IDs or whatever. I'm sure some of them do that, because why not.
https://bit.ly/19y8wyr+