[…] there is some reason for setting up the zero segment in the cases where it is possible to do so. The recognition of the zero shows that in these positions one can distinguish the grammatical category (and meaning) marked by the morpheme in question from that marked by its absence: 'I have cut' is distinct from 'I cut' just as 'I have taken' is from 'I take'.
- Harris (1951), a pag.336

[…] there may be situations in which we wish to say that a portion of speech which contains an observed segment is represented by absence of a linguistic element. That is, we may take a non-empty stretch of speech and say that it has no independent descriptive status and is in itself represented by no linguistic element, i.e. by a voided, or zero, element. All the cases in which this can be done are cases of partial independence, and the effect of this technique is to change these to cases of complete independence.
- Harris (1951), a pag.337