A simple TextBox can display text without or less formatting i.e it
can display all of its Text in same formatiing. Like Bold, Italic
Uderline or combination. You can give it fore color and also change
font. That's it!
On the other hand, RichTextBox facilitatets us to format different
parts of text differently. Like 1 Area of text is bold other is normal
while another is Bold + Italic+ Underline. You can alo put bullets in
RichtextBox, Also set Fore and Background color of text. Also
RichTextBox can display tables while Textbox can't. So there are many
scenerios where you do need this type of formatting and a simple
TextBox is of no use.
Exact examples of these 2 are Notepad (TextBox) and Wordpad (RichTextBox).