SHALL has been interpreted/clarified by US courts as not being a fancy MUST or REQUIRED that many people were taught it to mean, but SHOULD still has it's purposes, e.g., to provide contractual flexibility in development, i.e., a MUST/REQUIRED requirement was more challenging or complicated and took up more time/resources than anticipated, so SHOULDs can be trimmed due to contingencies.

Another example may be a lightweight implementation of a spec in a limited and/or narrow environment, which remains technically compliant with full implementations of a spec but interaction with such a limited/narrow environment comes with awareness about such limitations.