The author doesn’t go so far as to argue for greater use; she just defends their existing use. And anyway, we’re only interested in the purpose of P3 specifically, and the author doesn’t make any recommendations in P3. She just counters the critique that custom illustrations distort things.

The objection raised in P2 is that custom illustrations are misleading. P3 starts off with a variant of that objection: custom illustrations distort the issues. The author then replies, arguing that custom illustrations don’t distort; they just emphasize relevant details and leave out irrelevant ones.

The author isn’t arguing that textbook illustrations are bad—she’s arguing that custom illustrations are different from textbook ones in ways that make them particularly helpful and that don’t distort things, despite what some critics say. Her point isn’t to make us hate textbook illustrations; it’s to make us like custom illustrations. So even though she notes in P3 that textbook illustrations aren’t ideal, she only does so as a premise to support the real purpose of P3: to defend custom illustrations against critics.

Descriptively accurate but not the primary purpose. The author briefly describes a controversy with custom illustrations but then launches into an argument about why the naysayers are wrong. Her point isn’t to explain why custom illustrations are controversial—it’s to show that the controversy is unfounded and that custom illustrations are actually quite helpful.

Descriptively accurate but not the primary purpose. The author describes these differences to support a larger point: rather than distorting anything, custom illustrations just remove certain unnecessary details (ones that textbooks illustrations would include), which makes custom illustrations helpful for court cases.