If the postgres database is recording business transactions, you don't want to cause your business to stop being able to take credit cards because you generated a report.
Analytical queries (like "find the average sales price across all orders over the past year, grouped by store and region") can be 100-1000x faster in an OLAP database compared to Postgres.
You'd likely want a data warehouse if you also need to report on data that isn't in your prod database (e.g. stripe, your CRM, marketing data, etc.).
If setting up a data warehouse, etl, BI, etc. sounds like a lot of work to get reports, you're right, it is.
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