In addition to that the blog post lays out pretty clearly it’s for training:

> We use the constitution at various stages of the training process. This has grown out of training techniques we’ve been using since 2023, when we first began training Claude models using Constitutional AI. Our approach has evolved significantly since then, and the new constitution plays an even more central role in training.

> Claude itself also uses the constitution to construct many kinds of synthetic training data, including data that helps it learn and understand the constitution, conversations where the constitution might be relevant, responses that are in line with its values, and rankings of possible responses. All of these can be used to train future versions of Claude to become the kind of entity the constitution describes. This practical function has shaped how we’ve written the constitution: it needs to work both as a statement of abstract ideals and a useful artifact for training.

As for why it’s more impactful in training, that’s by design of their training pipeline. There’s only so much you can do with a better prompt vs actually learning something and in training the model can be trained to reject prompts that violate its training which a prompt can’t really do as prompt injection attacks trivially thwart those techniques.