Hi Jeff,
Using the CR and LF keys in all combinations was useful when using a manual typewriter.
CR and LF - The typist pushed the carriage return lever to return the carriage to the start of the line and move the paper up to the next line.
CR only - As Wikipedia stated, the only way to underline, cross out, or even “bold” text is to return the carriage to the start either without the linefeed (by using a carriage release button) or using the carriage return and linefeed and then turning the platten knob to roll the paper back one line. (The platten is the cylinder, sort of like a rolling pin, that the paper wrapped around which provided a firm backing so the keys could strike the paper without the paper tearing.)
LF only - rolling up the paper by turning the platten was useful if you needed to use whiteout to correct the text. You’d then roll the paper back to the original line, hit the backspace key, and correct the text.
There was also a button on the platten knob that allowed the paper to be rolled up part of a line which was useful for a footnote number, subscript, math formulas, an “*”, etc. Rolling the platten without using this knob moved a fixed distance, stopping at a “bump”. There were, give or take, 20 lines per platten rotation. (Just a guess, but you know what I mean.)
http://site.xavier.edu/polt/typewriters/tw-parts.html gives a picture of the typewriter and names its parts.