Linked from
The 11 pages that link to Carrier wave, each with the reason it gives.
Frequency modulationBroader topic: FM varies this waveform’s frequency to encode the message.
Signal modulationRelated: Modulation imposes the message on this waveform for transmission.
Radio transmitterRelated: It provides the signal that modulation changes to encode a message.
AM broadcastingRelated: AM broadcasting varies this wave’s amplitude to encode the audio program.