RBT(Risk Based Testing) in Agile Context

Risk Based Testing comes into the picture when you have lot of test cases to prepare/execute and the time for the same is very limited. Hence the need to prioritize the tests you conduct based on various dimensions of risk like business, operational, external etc. Yes you are right when it comes to individual agile teams they plan every task and since the stories teams commit are small enough to be completed within a sprint, so the functional testing also should be completed within a sprint. Here the scope of using RBT for functional testing is very limited. However if you have multiple agile teams for a single product usually you will have a separate team which takes care of integration, end-to-end system testing and performance testing. Here in such teams typically they use RBT for prioritizing the features for which tests should be performed

In SAFe context there is a system team at a Program level which is a special ART( Agile release train) team that typically provides assistance in building and using agile development environment infrastructure. This includes continous integration, test automation framework and end-to-end system testing. Here typically since the quantum of test cases to be executed is huge and time again is limited one can think of using RBT to prioritze test case execution/automation here. 

The need for RBT is derived from the type of testing one needs to perform and the scale at which it is to be performed