When you protect your document, you can also specify that only specific sections of the document be protected.
If you didn't define any controls for the form, then all the information still remains protected, and you end up with a document that cannot be changed. The idea behind forms is that you can specify the information to be entered in a form, using special controls, and then you protect the form so that you can only enter data by using those controls all other information in the protected portion of the form cannot be changed. This is done through Word's implementation of its forms feature.
Word allows you to protect parts of a document so that they cannot be changed.
