This is primarily to stop CSRF attacks, since otherwise an attacker can put <img src="phabricator.com/auth/settings/enroll/?providerPHID=xyz" /> on cute-cat-pix.com and get you to send yourself some SMS enrollment text messages, which would be mildly annoying.
We could skip this step if we already have a valid CSRF token (and we often will), but I think there's some value in doing it anyway. In particular:
- For SMS/Duo, it seems nice to have an explicit "we're about to hit your phone" button.
- We could let installs customize this text and give users a smoother onboard.
- It allows the relatively wordy enroll form to be a little less wordy.
- For tokens which can expire (SMS, Duo) it might save you from answering too slowly if you have to go dig your phone out of your bag downstairs or something.