Why must a modem to be used to transmit binary data through a PSTN?

A modem is necessary for transmitting binary data over a Public Switched Telephone Network (PSTN) because it converts digital signals from a computer into analog signals that can travel over the telephone lines. The PSTN is designed for voice communication, which uses analog signals, so the modem acts as a translator between the digital data and the analog format required by the network. Additionally, the modem ensures that the data can be reliably sent and received, handling modulation and demodulation processes effectively.