Incognito fields work exactly like regular PlainText fields, except that they can be configured to be
hidden for element editor forms.
A good example use case for Incognito is whenever you need a field that shouldn't be editable via the CP – for example, if you want to save data from a feed or external API to a field on an entry, without authors being able to edit that content.
It's also possible to override the rendering mode on a per-field basis using custom Twig logic. A good use case example is a field that is
readonly for all users except admins – or a field that's only editable for new entries.
Incognito fields are completely compatible with PlainText fields and can be converted to and from the latter without data loss.