I mean, why have the ability to generate weak keys to begin with? Serious question, I've never done this sort of work. What sort of utility does that bring to the procedures and workflows one does in that line of work?
If you didn't know better, you'd think this had been done on purpose by some three-letter agency.
Testing could be a reason.
To be clearer, I’m not sure these keys were generated, but being able to use weak keys is something you may want to do for testing.
What testing would require it? I've done plenty of testing setting up internal CAs & HSMs, and none of it required weak keys. Generating a key with the same parameters as a real key is trivially easy, and has the benefit of testing your key generation at the same time!