In BGP， we can use Route Reflector and full mesh. Why should we use these？

The rule using for IBGP loop prevention is described as bellow：routes learned from IBGP peer will not be advertised to any other IBGP neighbor. So full mesh or route reflector is used to make sure all the BGP routers within the AS can learn the advertised routes.Route reflector relaxes the IBGP loop prevention rule by reflecting routes from client to other clients and non-clients and from non-clients to clients.