By allowing the usage of connection parameters in the SQL connection string, we can pass useful parameters for i.e. Google Cloud SQL connection (using https://github.com/GoogleCloudPlatform/cloud-sql-jdbc-socket-factory for example), that would look like this :
Currently it isn't possible to properly add connection parameters (except through an ugly hack in JDBCDriverConfig.constructProtocol()).
As far as I know, using a connection string with the aforementioned Cloud SQL JDBC Socket Factory is the only way I found to connect to Cloud SQL. Outside of using Cloud SQL, I believe this feature may be useful for some people that need to inject some parameters to their connection string to the database.