Question about the Sentence_Break property

[Karl Williamson]

UAX 29 says this:

Break after paragraph separators.
SB4. 	Sep | CR | LF 	

Why are CR and LF considered to be paragraph separators?  NEL and Line 
Break are as well.

My mental model of plain text has it containing embedded characters, 
which I'll call \n, to allow it to be displayed in a terminal window of 
a given width.  Not all text is like that, of course, but there is an 
awful lot that is.  This rule makes no sense to me.