A hidden form field can be useful for saving returned id values, particularly as a means of passing them between multiple submit actions. However Sitecore does not provide a hidden field type out of the box.
FuseIT S4S provides a hidden field type automatically, or you may have your own implementation. S4D can also optionally provide a hidden field via the Status page.
Under General Settings, an existing hidden field implementation will be shown if found. If none are found, a button will be shown to Install S4D hidden field - click this to create a hidden field type in the master database under System/Settings/Forms/Field Types/Basic:
The new field type can then be added to experience forms.