I read once that the OSS agents preferred 32s over 380s because they were (1) more accurate and (2) more reliable.
As far as the Walther PP and PPK are concerned, they were designed around the 7.65mm Browning (.32 ACP) round, and they are very sweet shooting little pistols in that caliber.
The are not nearly as pleasant to shoot in 9mm Kurz/.380 ACP.
Reliability however is often more of a challenge for a pistol in .32 ACP as it uses a semi-rimmed case and can be subject to rim lock if the user isn't careful to ensure the rim of the cartridge is in front of the one below it in the magazine. Although to be fair this is usually only an issue with rounds that have a shorter than normal OAL.
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In terms of performance, the .32 ACP is out of favor in the US, where we barely accept the lowly .380 ACP.
But in a PP where you can launch a 60 gr XTP at around 1000-1050 fps, it actually performs as well as the .380 ACP, with 12" penetration in gel and relabel hollow point expansion. It just don't expand as large as a .380 ACP hollow point and I don't feel any more under gunned with a PP in .32 ACP than I do with a PPK/S in .380 ACP. With a 73 gr FMJ, the .32 ACP also penetrates adequately. And in a PP series pistol you get one more round in the magazine with .32 ACP compared to .380 ACP.
With either the .380 ACP or .32 ACP, the three things that count most in stopping an assailant are:
1) shot placement;
2) shot placement; and
3) shot placement.
The major shortcoming of the .32 ACP, IMHO, is that it takes an even larger hit than the .380 ACP in short barrels, so you really need to stay with the 3.9" barrel of a PP, or at worst, the 3.4" barrel of a PPK or PPK/S rather than going with one of the shorter sub micro .32 ACP pistols.