Hi Sheila,

All Forms fields have variables associated with them. These variable allow you to access the content inputted into a field (field variables) or other information associated with the instance of a process (process variables). Using variables you can do things like fill out the metadata of a form when you store it into the repository, make decisions as to how an instance of your process runs, and create dynamic content in emails.

Regardless of how you use them, at the very base level variables keep track of all of the information about a given process and allow it to run. Deleting a variable makes it so that the field it belongs to is no longer usable (Forms can't keep track of the information put into that field without its variable!). As such, deleting a variable removes the field as well. 

For more information on variables in Forms, look at this section of the help files.