I have been tasked with designing a sign in flow, but I have noticed that many companies hide their site navigation during their sign in/sign on flows. I've seen quite a few articles on designing these flows, but none of them mention site navigation (take a look at Login Form..., 18 UX Design Tips..., UX Login..., 10 tips..., and Designing a...) You don't have to click on any of those because they all have the same basic information, none of which includes "hide your site navigation"
The examples that I have looked at include:
I have a lot of guesses, so let's steer away from speculation if possible, I'm looking for behavioral reasoning or data/numbers on conversion rates.
Edit: So in order to get a user to complete a task, they remove navigation because it's perceived as 'noise'? Is this a practice that is done with tasks? Complete any task on this site. Send a message, reply to a comment, change your avatar, ask a question. Does navigation disappear? No.
Good UX practices make things easier for the user, dark UX practices make things more difficult for the user in order to make a sale or convert. Preventing the user from freely moving about the site in order to get them to sign up is dark UX. I would not feel like a responsible UX designer if I told the developers to remove all navigation from a sign-up path if the only reason was to get them to do it.
However if users actually find it helpful, I'm sorry, but I need proof and not guesses. (qualitative: 79% of users say "I HATE navbars when I'm signing up" or quantitative: "sign ups went up 12% when we implemented XYZ")