with all those scripting questions I would truly recommend you attend an ICM scripting class ;-)
Coming to your concern, schedule target is used to send calls to a remote destination, usually a key system or a Pabx that is not providing any information back to the ICM router, but it is capable of accepting the call, it is what in ICM terms is called the "poor man ACD", ICM is providing an end destination and holding the call up until it gets accepted on the other side.
It might even be you have some remote TDM where you have already all your call handling defined and want ICM only to perform as a SUPERACD to network your different ACDs together, it is usually not used in an IPCC or HIPCC environment.
Announcement is a VRU announcement and peripheral is an ICM peripheral clearly.
As to why the destination can't be an agent to agent node or a SG or a SVC we would need to see your script, if you need assistance with one specific script or a set of them Advanced Services just started offering Dev Support Services also on ICM Scripting, they require only a minimum of ICM Scripting experience and a low fee as an annual subscription and provide assistance on similar tasks and questions.