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.