In the Form Designer property panel, there is an option “CSS Class ” for you to choose which css class to apply.  Shown as the screenshot below:

You can add additional custom class names to this dropdown list by adding  Extended List Item to the following path:

/sitecore/system/Modules/Web Forms for Marketers/Settings/Meta data/Css Classes

The benefit of adding custom CSS class to each field is to have the flexibility for web designer to change the form style according to the website’s theme.