Why do you use salt to melt ice in the winter but freeze homemade ice cream in the summer?
Why is salt used with ice to make ice cream, yet you pour salt on icy sidewalks to melt the ice?
Back to the simplicities of chemistry. Salt lowers the freezing point of water, typically to the mid-20s Fahrenheit. If the air temperature is greater than the mid-20s, ice sprinkled with salt will melt.
In an ice cream maker, you want to rapidly lower the temperature while whipping air into the cream mixture using the crank. The water dripping from ordinary melting ice remains at a constant temperature of 32 degrees. The water from a melting ice and salt mixture, however, drops to the mid-20s due to the lowered freezing point. The water is what comes in contact with the canister containing the ice cream, so the lower its temperature, the quicker the ice cream freezes and the sooner you can stop cranking and enjoy.