It allows twitter to easily track which links you're clicking on, which some might consider to be a violation of privacy.
On mobile, t.co makes it so that you end up always launching a browser before launching the appropriate application (i.e. YouTube)
When you copy and paste and send a link to someone, they have no (easy) way of knowing what you're sending them without description or visiting, and it gives Twitter a mechanism for tracking who you send the link to. (One could posit a "malicious" tracking twitter where they serve up t.co links to people which are dependent on the logged in user to track that user's social network... fortunately I don't think this has happened yet.)
It allows twitter to easily track which links you're clicking on, which some might consider to be a violation of privacy.
On mobile, t.co makes it so that you end up always launching a browser before launching the appropriate application (i.e. YouTube)
When you copy and paste and send a link to someone, they have no (easy) way of knowing what you're sending them without description or visiting, and it gives Twitter a mechanism for tracking who you send the link to. (One could posit a "malicious" tracking twitter where they serve up t.co links to people which are dependent on the logged in user to track that user's social network... fortunately I don't think this has happened yet.)