Keep Transport Lag low so your Recovery Point Objective can be met. You want transport to be quick so the Standby has all the information it needs to become the Primary (with minimal data loss) should the Primary go down.
For more information on Recovery Point Objective (RPO), refer to the Oracle Database Help Center.
In Oracle versions prior to 11.2 some configurations experience inconsistent or misleading data on the Transport Lag. This is documented as Oracle Knowledge Article [ID 1299000.1].
An Example Scenario: Outstanding Logs are speedily applied. Therefore you would expect the Transport Lag to be low, but the Transport Lag is increasing. The Transport Lag data is misleading. Disable the Transport Lag Alarm and use the Outstanding Logs To Ship Alarm to check for bottlenecks.