When you need to transfer or stream data from one device to another there is always some dilemma what technology to choose. You can pick many ways of doing so. For instance these can be USB, USART, IRDA, RF, WiFi, Bluetooth, SPI, I2C, etc. but when you are using small micros like Attiny85, you start thinking about code size and simplicity. Another thing is availability. If you need to send data from computer or portable device, options become even more limited. WiFi or Bluetooth is too expensive comparing to overall project cost, USART is thing of past unless you use adapter. Jari even tried optical screen flickering method, which appeared to bee to slow and not reliable. So he thought what interface is always there and doesn’t require many pins. So he remembered modem idea.
Audio line is almost on every consumer devices including PC, smart phones. It can also be easily implemented on any microcontroller. So he started playing with data transfer overs audio line. To send data you need only one wire (and pin). Sending data with sync signals once in a while ensures very reliable data transfer up to 12kbps rate.