The ‘Field button group’ is an input field with a button next to it, for example used for search fields that are on pages or part of blocks.
Use this group when you need an input field combined with an action. A good example is a search field, but it could also be used for signing up to a newsletter, or similar purposes.
Don't use as part of a form, use the forms modules for that.
Notice that in mobile viewport, the group is seen as one module and share the width.