See http://in.relation.to/2017/02/16/hibernate-connections-cloud/ for an overall why of that feature.
tl;dr; for cloud environments when the database might be present after the application bootstraps.

Note that Hibernate ORM is already partially supporting such case, this issue is about improving the situation:

• fix issues with pools not friendly towards the DB not being present at boot time HHH-11517 Open
• delay schema generation calls to when the DB is present
• explore ability to extract JDBC environment information from DB connection in a delayed fashing
• allow the connection provider to circuit break when the database is not present