A physical sound unit is an allophone . Allophones are non-contrastive. They may also offer an explanation of why the process takes place. The explanation lies in the phenomenon of assimilation , the natural process by which a sound changes in order to become more like another sound. We anticipate a sound and it changes another sound.
Vowels become nasalized because they assimilate to the nasality of a following nasal.
FACT 9 Phonological rules describe the regularities of the sound system of a language.