Radio Promos were more important to the show in the begining because the show was cross promoted with other shows that were on the onine radio station that Practical Lunacy was on. These days, they are not so important, but Luna still enjoys making the promos, and they played during the music breaks. If you listen via podcast, you'll hear only the promo after Luna says they're taking a music break. Sometimes they get retired, and replaced, either due to hosts changing or a particular interesting conversation being had on air.
That all said, you can listen to or download the currently running promos here: