Why would you do a whole new expensive, time-consuming training run if you can just tweak some numbers to get the result you want?

I think your use case of incorporating novel data probably calls for training, but I'm sure there are use cases where you can tweak the weights to get what you want (say I wanted to tweak the model to be less sycophantic, or found a way to shrink it by removing weights associated with a language that I don't ever use)