Until now, the formulas defined on joined datasources were not available within the join itself. You had to clone these formulas to reproduce them in the join. Because of these duplicates, it was painful to maintain large projects with many joins.
When the new option "include datasources formulas" is enabled (in a join datasource configuration), a formula created in a joined datasource is available in the join datasource. You can use it within new formulas, directly in your dashboards, or in your data-preparation steps.
This option is not enabled by default on existing joins as it might create name collisions between your existing formulas on the join and the joined datasources formulas.