These are functions for dealing with packing and unpacking data to and from the network representation. Normally, the Verse API hides all data pack/unpack operations, delivering already-unpacked parameters directly to an application's callback functions. However, object method calls require extraneous information that is not buffered by the API, so to properly unpack a call's parameters, the application needs to be in control.
Also, the separation of packing a call from actually sending it, can help improve performance if a single call needs to be sent multiple times, since it makes it possible to buffer the packed representation and re-use it for any number of calls.