Hi DJ,
I can partially answer your query.
When you do a POST request using this URL, https://<SMPServer>:<ProxyPort>/odata/applications/v1/com.sap.poc.restapi/Connections
on success, you get "201 created" status message along with Application id created in RESPONSE header. right?
Now to get backend details (SOAP WS),you have to call this URL http://<SMPServer>:<ProxyPort>/<ApplicationID> where you have to pass above returned Application id request header of GET request. Check this
In case of REST API app development approach, SMP acts as a proxy. In addition to this, you can trace device logs, total number of device registered in SMP, security, push notifications.
Rgrds,
JK