If someone has something to say, s/he will probably just say it. Why does s/he, instead or in top of that, use a quotation of someone else?
1. In order to add authority to his/her statement (since X said it, it must be true).
2. Because X has already said something very significant on that subject and one cannot ignore it.
3. Because X has already said something very significant on that subject and one wants to improve on it.
The third case might be very promising. If an author quotes a passage and then interprets it in a forced way, this might mean that s/he had to quote it (for instance, because it was the standard text on that particular theme), and hence wanted to force its interpretation into his/her own one.
I am working on the principles of tantra and prasaṅga in Mīmāṃsā. They seem to have been re-interpreted by Śabara. Śabara, however, opens his discussion with a verse which –he says– is used as an illustration (udāhṛ-). The verse is immediately followed by Śabara's own reading of it (which, I believe, does not correspond to its original meaning). Hence, I think that Śabara had to mention the verse, but then domesticates it into his own view. In this way, he conveys the idea that his innovation was already common sense among Mīmāṃsakas.
The verse says:
sādhāraṇaṃ bhavet tantraṃ parārthe tv aprayojakaḥ |
evam eva prasaṅgaḥ syād vidyamāne svake vidhau ||
And Śabara writes:
sādhāraṇaṃ bhavet tantram […] parārthe tv aprayojaka iti. yaḥ parārtham utpannas tadartham eva cānuṣṭhīyamānaḥ parasyopakaroti, sa parastasyāprayojakaḥ.
It seems to me (and before me to Bronkhorst, see Bronkhorst 1986) that the interpretation of aprayojakaḥ as a noun referring to tantra instead of an adjective referring to prasaṅga is far-fetched and, therefore quite telling. Śabara probably had before his eyes/ears a verse distinguishing tantra and prasaṅga in a certain way, wanted however to distinguish them in another.