1) you travel a lot and sometimes may need to mail your passport off to get a pre-travel visa from some consulate, while you are still outside the USA. This way you can send off one passport to get the visa for your next country, while keeping a valid passport with you while you’re abroad.
2) You need to travel between Israel and countries which have laws against visiting Israel (Historically, GCC countries). This way you can always present whichever passport doesn’t have Israel’s entry and exit stamps on it.
I’ve had two valid US passports at the same time, and I’m just a random nobody American.