I would say BQ is for analytical queries. Not restricted to batch, as most BQ queries are interactive.

The use case here is when you have some relatively small amount of data, typically some dimension or reference data, and need some processing beyond what BigQuery provides, like MakeValid here. Your main fact data is hopefully valid, but reference data you get from public sources might be not. This "bridge" lets you use PostGIS much larger functionality for this data.