Please refer to the attached gif file.
Let's take for instance demo application with its default document, with text "RadRichTextEditor for WinForms" at the beginning in font Calibri of size 28. Suppose the user wants to add A, then empty line, then C in single line, followed by the rest of the document, and to have these three new lines in Times New Roman in size 10.
If there is no empty line, but letter B in middle line, then user can set caret to document start, set the font and size, and type A<Return>B<Return>C<Return>. And it would look fine (and even then, if we set caret just after B and press Return the new paragraph will have spacing after set to 7.5 for no apparent reason).
If we want an empty line instead of B in the line, then there is no simple way to do it. Typing A<Return><Return>C<Return> would produce a huge gap between A and C (we still first set the caret, and font and size). One way to accomplish what we want is to select all new text, set font size to something other than 10, and then back to 10 (which is also weird to have to do).