You have a need for a tool allows you to see the UI “as the user does” so you can respond to support requests and maybe someone thinks it’s easier to just copy the cookie.
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.
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.
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.