I've never used these two commands since the FOR command sort of makes them unnecessary. They seem to operate exactly like HP RPN ISG and DSE, but the manual states that "they are not looping instructions." Why would it say that? Why else would you use these? Why are they even included in a non-RPN calculator?

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They seem to operate exactly like HP RPN ISG and DSE, but the manual states that "they are not looping instructions." Why would it say that?

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Because they aren't? (And I'm not trying to be snarky.) In the strictest technical sense, ISG and DSE are conditional skip instructions, which are great for implementing loops, and indeed that is their primary purpose -- but the skipped instruction doesn't have to be a GTO.

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Why else would you use these? Why are they even included in a non-RPN calculator?

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I vaguely remember using ISG/DSE for purposes other than looping in some programs I wrote a very long time ago. I'll try to dig up an example, but don't hold your breath. (It's all written down on paper... somewhere.)

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using ISG/DSE for purposes other than looping

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Yes, they are sometimes used in RPN just to increment or decrement a variable without looping, to save a byte or two, or avoid using the X register, but this seems rather dumb in a non-RPN calculator.

In the discussion about this program, http://www.hpmuseum.org/cgi-sys/cgiwrap/hpmuseum/archv018.cgi?read=128554 , the usage of ISG as a mere "increment register" instruction is mentioned. While I used this technique, further analysis by Jeff O. showed it was not the best choice.