Line coding for a modulation order that is not power of 2
Another reason to encode to $3$ level is that the occupied bandwidth is less.   This is often driven by an emission mask constraint  for transmitter certification. To satisfy this constraint,  the tradeoff is made on symbol distance.   It is however a costly tradeoff affecting receiver sensitivity because the multi level scheme is done post detection where every factor of $2$ increase in throughput costs $6$ dB in sensitivity.   Other schemes like QAM cost only $3$ dB in sensitivity when doubling the number of bits per signaling element.  With the multilevel FSK, you do however retain some of the advantages of an FM strategy in  terms of the CIR performance of the system.