In this verse, the Quran responds to the Meccans' demand for a miracle by pointing to the existence and clarity of the scriptures that came before it (the Torah and the Gospel).
Surah 20:133:
And they say: If only he would bring us a miracle from his Lord! Hath there not come unto them the clear proof of what is in the former scrolls?
The verse asks: "Has there not come to them the clear proof (Bayyinah) of what was in the former scrolls?"
A "clear proof" must be recognizable and uncorrupted. You cannot use a "corrupted" or "lost" document as evidence to prove a new claim.
By pointing to the "former scrolls" as the miracle that proves the Quran, the Quran is vouching for the integrity and availability of the Bible in the 7th century.
Classical commentators (like Ibn Kathir) identify the "former scrolls" as the Torah and the Gospel.
This anchors the Quran to the historical manuscripts of the 7th century.
We possess the manuscripts that were in circulation during the 7th century (e.g., Codex Sinaiticus). They match our modern Bibles. If those manuscripts are the "clear proof" for the Quran, then the Quran must match their contents.
If "Document A" (the Bible) is the proof for "Document B" (the Quran), then Document B cannot contradict the core message of Document A.
The "former scrolls" explicitly teach the Sonship of Christ and His death on the Cross. Since the Quran denies these, the "former scrolls" actually provide "clear proof" that the Quran is not from the same source.
This verse essentially makes the Bible the Judge and the Quran the Defendant. If the Judge's records (the former scrolls) don't support the Defendant's story, the case is dismissed.