I think it’s best to think of the algos as having 4 basic shapes:
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Stack - (eg algo 4) with this configuration the modulation of each operator is gated by the operator below it. So the effects of B2 is only heard when modulation B1, which is only heard when it effects A, which is only heard when you modulate C.
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Branch - (eg algo 1) with this configuration multiple sources modulate the same operator. So B1 can still modulate C even if A’s modulation depth is turned all the way down.
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Root - (eg algo 3) with this configuration the same source modulated many operators simultaneously. A modulating C, B1, and B2 at the same time allows for a stacking of timbres in addition to modulation.
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Carrier only - (eg algo 8) with this configuration the operator is directly part of the mix with no source modulating it. The digitone doesn’t have a great example of this but algo 8 comes the closest. Perhaps the easiest use case for such an algorithm would be to make an organ sound or mixing some fundamental back into a crazy sound.
All algos on any FM synth are combinations of these parts. So I tend to choose my algorithm based on whether I need to nest the modulations in a stack, have them mix together in a branch, or have sounds layer with a root.