There are some cases where adding a read replica can be helpful at almost no extra overhead - for instance if your product has something like a stats dashboard

There are some cases where adding a read replica can be helpful at almost no extra overhead - for instance if your product has something like a stats dashboard you'll have some heavy queries that are never going to result in a write after read and don't matter if they are sometimes a few ms or even a few seconds or tens of seconds out of date. Similarly if you have analysts poking around running exploratory queries, a read replica can be the first step towards an analytics workflow/data warehouse.