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Someone else can confirm, but i believe this is to confirm availability of the segments for each datasource (i.e. for the Availability column in the web console Datasources page)

And when segment metadata queries failed, it will block broker startup?

WARN [DruidSchema-Cache-0] org.apache.druid.sql.calcite.schema.DruidSchema - Metadata refresh failed, trying again soon.

yes, Broker startup does block until metadata is initialized