Base field descriptions were both used as further explanation on entity forms and in database schemas. Unfortunately the dual use of the descriptions makes them fit neither use case. Too technical descriptions are problematic on the UI (see #2350013: Descriptions for Promotion Options are too technical), while too user friendly descriptions are not a good fit to describe the database schema. Also base field definitions translate the description on the fly and therefore using that in the database schema results in schema changes reported in the generated entity schema on multilingual sites depending on the language used.
To separate these two use cases and not use translated values for schema descriptions, we don't use the base field description anymore in the generated entity database schema.
