According to some random internet statistics 90% of the people experience earworms.e.g. getting a song, or usually part of a song, stuck on repeat playing in your head. Sometimes it can also be just a word, a thought or memory. But why does this happen?
For music there are some theories how this might be evolutionary (aren't anything we are). Before written language, even spoken one there has been music. Or at least some rhythm and melodies that have conveyed meaning and helped us remember things. Being able to repeat and recall that information has been important part of our learning and communication since the early days.
It might have been useful back then, but nowadays it seems it's always something useless, even annoying information that keeps repeating. Of course it could be brains way to "exercise" to be better at remembering and recalling information. It could as well be a bug in our "programming" that just occasionally ends up in an endless loop for no apparent reason.