Ordainarily pressing the "tab" (tabulator, ⇥) key will navigate between either form elements or all interactable elements of the page.
This is an accessibilty feature permitting use of a page without a mouse.
However, it can be desireable to permit entry of literal tab characters into text entry boxes.
Pro: You can actually type tab characters.
Con: Becomes impossible to "tab out" of the text area, breaking accessible navigation.