A pipeline needs more than a sequence of commands: someone must schedule it, see what happened and know when it failed. Dagster coordinates that work in Lagom.
Separate places to run
Staging is the rehearsal space; production is where the real pipeline runs. Each has its own Dagster instance and its own Postgres metastore, which records the history of runs. Changes can be exercised in staging before they reach production.
The cost of isolation
One shared instance would use less resident memory, but it would cross the boundary between environments. The accepted design spends more resources to keep that boundary clear.
Dagster coordinates dlt and dbt in each environment; it does not replace their roles. Collection, transformation and scheduling remain distinct pieces.