I expect it's for a drop-down menu in the dashboard.
If your pg_exporter_last_scrape_duration_seconds metric doesn't have any
label "kubernetes_namespace", then that Grafana query isn't going to find
any values for that dropdown.
But if the variable has an empty value, then any dashboard query of the form
pg_whatever{kubernetes_namespace="$namespace"}
will match all metrics with {kubernetes_namespace=""}, which is the same as
metrics without a "kubernetes_namespace" label.
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