You misunderstand. "Why twitter does redirections" is "why does twitter follow Location headers to get unfurl info / metadata", not "why does twitter have t.co", and the reason is because marketers use bit.ly etc, so twitter has to follow those redirects.
Marketers/users cannot view t.co metrics, but even if they could, they'd want to use their own url shorters anyway I'm sure... so twitter has to have the t.co previewer follow arbitrarily many redirects.
