Carrying over another comment from a CL:

Today, site developers are able to test signed exchanges by getting a TLS certificate from a trusted CA (hopefully with a different key than their production server) and testing the rest of the packaging flow, such as SCTs and OCSP responses. Without this flag - that is, with strict enforcement of the extension - there's no way to test locally until CAs support this, short of --ignore-cert-errors-spki-list, which would mask SCT and OCSP errors, preventing testing of the very thing it's supposed to be testing.