To connect signals from A to B one may use, for example, two parallel wires. In fact, this does work up into the HF-range if we keep our eyes on the impedance of this two-wire line. Disadvantageous is, however, that the field propagates around the wires and that everything coming close will have an impact on the transmission properties.
Waveguides, on the other hand, carry the electro-magnetic field inside a metal tube. What happens externally has no impact on the field. The disadvantage here is that there is a minimum frequency from which the energy transport is in fact possible. Moreover, such a conductor is very stiff and may not be bent easily