There is a special feature in sometape decks and cassette tapes that allows the tape deck to stop playback or reverse the playback when it’s reached the end of the cassette. People who have grown up listening to cassette tapes will be all to familiar with the sound of the tape when it reaches the end - like a loud CLUNK. What next would occur is that the cassette would automatically reverse and start playing the other side.
Cassette adapters aren’t real cassettes (well at least not on the inside) which means that inner mechanisms are completely different. Cassette adapters feature a special mechanism made from a series of gears that ‘tricks’ the head unit into not stopping or reversing. Without this device your cassette adapter would continue to stop and reverse and wouldn't be able to play a continuous loop.
Some people have problems with cassette adapters that do stop and reverse, in this case the adapter is no doubt faulty and this could be due to the gear mechanism being broken.