The AAC was designed to replace the MP3. For the same bitrate (which is measured in kbps) and the same MP3 file size, the AAC format is more stable and has higher quality, producing a more crystalline sound.
When you contract a conventional audio streaming service you require a high bandwidth (128 Kbps) to transmit your radio station with FM Stereo or CD quality. In addition, it implies a high cost of maintenance of the service and the enormous possibility of a choppy signal when it is heard by users who have access to Internet with little bandwidth.