This Question is somewhat misguided, and seems to assume there is some special relationship between the input and output devices.
The problem can be broken down into 2 parts:-
How do you control the position of a servo.
How can you input a value into that program.
The second can indeed be done with a potentiometer and ADC but, depending on your ...
The potentiometer is used to signal the desired position for the servo. The Analogue to Digital Converter (ADC) is used to determine the pot position by reading the voltage between the pot legs.
If you know of an alternative way of determining the pot position you can use that instead of the ADC.
You can use another ADC instead of the MCP3008.