What is the difference between serial begin() and wire.setClock() for I2C communication?
Serial has nothing to do with Wire/I2C communication. The examples only have this, because mostly they use Serial out- and input for interaction in the example codes (like showing the data of an I2C sensor on the Serial Monitor).
If you don't want to use the Serial Montior/Serial communication, then you can leave all the calls to Serial out.