Why Export GA4 Data to BigQuery?

"Then coming on to the next point, which is you can create your own sessions and user properties. Now you can do this in the GA4 interface under Explorations."

"But there are two main limitations here. The first one being that you have to use Explorations, and the second is that you can only create dimensions. And when you've created these as well, these only live in your Explorations. You can't have these in other places, if you want to view them in dashboards or whatever. But obviously, with the raw GA4 BigQuery data, you can create these properties, and then you can use them wherever."

You can create custom sessions and user properties inside the GA4 Explorations interface, but those creations carry significant limitations. Two main limitations exist: creations must be made within Explorations, and only dimensions can be created. Created dimensions remain confined to Explorations and are not available in dashboards or other GA4 areas. Accessing the raw GA4 BigQuery export data enables creation of equivalent properties at the data level and supports using those properties across reports, dashboards, and other tools. BigQuery-based properties therefore provide broader availability, reusability, and flexibility for cross-report analytics, though they require BigQuery setup and SQL skills.

Read at Moz