F-16 SPI / Slew management - how did it come to be?

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Where can I learn more about the design decisions around how F-16 handles sensor slewing/SPI?
I find that interesting/different that by slewing a sensor, the entire flight plan is moved. Why do that? What were the decisions behind that?
Why wouldn't it be possible to simply note the location of where the SPI is pointing without moving all the nav points?