Usually, text boxes are used to position texts somewhere on a page. In this case, you need not to change the role Default.
Pay attention to the anchoring of the text box, as this has an impact on the reading order (the content of the text box follows the content of the paragraph on which it was anchored).
You can change the role of a text box in the task pane axesWord - Role Mapping.
If the text box is used as an illustration or decorative element, change the role to Figure or Artifact.
It is also possible to assign the role Formula to the text box.