specify an alternate signing or decryption key to use
control signing or encryption algorithms (but for metadata you control, it's advisable to control algorithms by using an extension to specify supported algorithms).
The two interceptor lists allow the much less commonly used profile interceptor injection points to be used. This is largely a Java-based way of doing very low-level sorts of “message rewriting” hackery that might otherwise be impossible to pull off. One use case for the inbound side might be picking up non-standard parameters in a SAML request.