Linked from
The 9 pages that link to Phonological rule, each with the reason it gives.
AllophoneRelated: Rules can describe where a phoneme's allophones occur.
Distinctive featureRelated: Rules can refer to features rather than list every affected sound.
PalatalizationRelated: Rules can describe when a language changes consonants through palatalization.
PronunciationRelated: Such patterns account for predictable pronunciation changes within a language.