Quoting the analysis in the ruling:

Authors also complain that the print-to-digital format change was itself an infringement not abridged as a fair use (Opp. 15, 25).

In other words, part of what is being ruled is whether digitizing the books was fair use. Reinforcing that:

Recall that Anthropic purchased millions of print books for its central library... [further down past stuff about pirated copies] Anthropic purchased millions of print copies to "build a research library" (Opp. Exh. 22 at 145, 148). It destroyed each print copy while replacing it with a digital copy for use in its library (not for sharing nor sale outside the company). As to these copies, Authors do not complain that Anthropic failed to pay to acquire a library copy. Authors only complain that Anthropic changed each copy's format from print to digital (see Opp. 15, 25 & n.15).

Bold text is me. Italics are the ruling.

Further down:

Was scanning the print copies to create digital replacements transformative? [skipping each party's arguments]

Here, for reasons narrower than Anthropic offers, the mere format change was fair use.

The judge ruled that the digitization is fair use.

Notably, the question about fair use is important because of what the work is being used for. These are being used in a commercial setting to make money, not in a private setting. Additionally, as the works were inputs into the LLM, it is related to the judge's decision on whether using them to train the LLM is fair use.

Naturally the pirated works are another story, but this article is about the destruction of the physical copies, which only happened for works they purchased. Pirating for LLMs is unacceptable, but that isn't the question here.

The ruling does go on to indicate that Anthropic might have been able to get away with not destroying the originals, but destroying them meant that the format change was "more clearly transformative" as a result, and questions around fair use are largely up to the judge's opinion on four factors (purpose of use, nature of the work, amount of work used, and effect of use on the market).

The print original was destroyed. One replaced the other. And, there is no evidence that the new, digital copy was shown, shared, or sold outside the company. [The question about LLM use is earlier in the ruling] This use was even more clearly transformative than those in Texaco, Google, and Sony Betamax (where the number of copies went up by at least one), and, of course, more transformative than those uses rejected in Napster (where the number went up by "millions" of copies shared for free with others).

... Anthropic already had purchased permanent library copies (print ones). It did not create new copies to share or sell outside.

TL;DR: Destroying the original had an effect on the judge's decision and increased the transformativeness of digitizing the books. They might have been fine without doing it, but the judge admitted that it was relevant to the question of fair use.