What's the purpose of defining timestamp_add_sql in all the adapters?

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It seems to be redundant with defining a <adapter>__dateadd macro implementation, and I can't figure out why it's needed. (I'm working on a new dbt adapter and am trying to understand the purpose.)

I did a Github code search to try to find references to adapter.timestamp_add_sql and all I could find were implementations in nearly all adapters, and tests code that references it. Is testing the sole purpose? Can we consolidate and try to use the dbt.dateadd macro in testing instead of requiring adapters to define this method to generate SQL?