Unlike DVDs, CDs don't have sector information. They're just a long continuous spiral of bits, so there is no easy way to tell exactly where on the spiral the head is pointing, especially as many CD players, when you tell them "go to here", will miss slightly.

To compensate for this, data CDs include intermittent data on the spiral that says "you are here", but audio CDs don't do this. The only way to ensure that you haven't either skipped over a piece, or duplicated a piece, is to rip the CD in overlapping chunks, and then compare the overlaps.

Some CDs are very scratched. Some CDs are CDRs with dye that hasn't aged well. Some CDs just don't read well (poor manufacturing tolerance, perhaps). Some drives silently return bad data on error. Some drives return a lot of errors towards the last tracks even when there is no problem. Drives vary on where they think tracks start and end (generally a uniform offset per drive). Sometimes, reading audio before the claimed start of the CD takes extra trickery. There are many more issues, besides. By ripping multiple times to verify and comparing to a global database of checksums (and in some cases doing error correction), you can be sure of getting exactly the intended audio.

Audio CDs and data CD-ROMs use different encoding modes.

Data CDs have an extra layer of error correction. Audio CDs have less error correction because small bit errors are not a big deal for your listening experience. Most CD players quietly interpolate over small errors in a way that you probably don't even notice.

I read the linked rationale and still don’t understand why this is even a problem.

As early CD players were intended for audio playback, your CD player can lie to the software. It doesn't tell if it's reading at the exact beginning of a sector (because timing information is multiplexed into the audio stream, and some CD players were not byte-exact when interpreting this), and it also doesn't tell you when it silently corrects some errors. Because for a human listening to the audio won't notice.

But of course it makes a difference if you want to make high-fidelity copy.

That's why there always have been copy apps (cdparanoia on Linux, EAC on Windows) that do overlapping reads, and then detect and compensate these problems.

I never understood that either, maybe redbook doesn't have a strict error correction like data CDs have? That's the only thing I can think of but that seems weird though.

Transferring from disc to system via a laser is analogue and error prone. What if there is a scratch? What if the cd wobbles?