Why are isolation transformers often included in the sound card interface of a station operating computer-based digital modes?

Connecting a computer and a transceiver directly together via audio cables can easily create ground loops. Because both devices usually have their own power supplies and grounds, slight voltage differences between their grounds can cause electrical currents to flow through the audio cables, introducing a loud 60 Hz AC hum or other noise into your transmitted signal. Isolation transformers solve this problem by magnetically coupling the audio signals between the computer and the radio without any direct, physical electrical connection. This breaks the ground loop and ensures that only clean, interference-free audio reaches the transmitter.