User postbit is the the place where your name and Avatar with your post count and other informations are displayed , it is the same place where you can update you current ISP and previous one by going into your profile settings thus making it displayed on every post that you make here achieving the same thing that you were using your signature for Plus I don't know why it is disabled here for everyone but it will try to make an educated guess that it reduces the clutter and if troops the readability of the thread drastically , so it is better to keep it disabled in favour of better readability of the content for the users
xenforo does not make it easy (i have an unanswered query on their support forum for this) to show additional postbit information on mobile devices. i also have signatures disabled on mobile web app. lesser screen space has its compromise and postbit information is one of the casuality. it does appear in user profile details. but of course, that does not solve the purpose as it requires opening profile in a new tab.