Hi Amber,
It all depends on your use cases as to what you want to do with HCP. If you have an SAP backend system where OData services are available, you would probably consume those ABAP based OData services directly in HTML5 applications built on HCP. This is something you are already familiar with.
Maybe you are looking to create some new applications and you have java developers who could help. You can use Apache Olingo (Java library) that implements the OData protocol and deploy it on HCP.
You can use the databases on HCP when you looking to store some data on HCP. You could use this when you are creating a brand new application on HCP (which does not have to do much with backend SAP systems). You can put up a Java application to deliver the UI and connect it with the databases on HCP. Another good example of having database on HCP would be to have an agile data mart in the cloud. You may have data available in different sources (cloud and on-premise). You can replicate the data from all these sources into the HCP database and then build applications which report on this data.
Cheers,
Murali.