How do I use subscriptions effectively? 

The top two most common and useful subscriptions:

1. Create one to monitor engine status. Everyone should have one that shows if an engine stops responding. Getting it sent to email or to a slack channel via webhook would work. https://docs.streamsets.com/portal/controlhub/latest/help/controlhub/UserGuide/Subscriptions/Examples.html#concept_fgy_fbz_ddb

2. Monitor job status. If the status changes to error it can notify you. 

There are many others but these two are the most common. 

You can monitor if a data sla is triggered. If you expect a pipeline to process 5k records per second, you can create a subscription that triggers if the pipeline processes less than 4k per second. https://docs.streamsets.com/portal/platform-controlhub/controlhub/UserGuide/Topologies/MasterDataSLAs.html#concept_zzj_5kh_2z

You might also want to get notified if pipelines change or get checked in as well.