This sounds like speculation. Is there evidence that Twitter has a tool that allows employees to see the UI as the user does?
The question was interpreted as “why do admin tools have these features” on account of that was in the first sentence, and not, as you may have imagined, a request for a twitter employee to explain all of their tools, or a request for a twitter employee to justify creating these tools.
Re:An update on our security incident
by onefuncman on Friday July 31, 2020 @04:26AM
(#24006797)
it's not copying the cookie, but there are absolutely third party UIs via a user's API key, it's not a huge leap of faith to assume Twitter has similar internally.
This isn’t the right way to do it, but given they work at Twitter I could imagine this isn’t the first big mistake they’ve made.