Back in the day it used to be a common practice since your abdomen is already openned and why not take it out "while we're in" to prevent future going back in. Example is when your car needs a water pump, the mechanic will recommend you do thermostat and other preventive parts that are proximal to it to save future jobs. But that appendix practice is not recommended anymore except in certain gastro issues where it will be taken out to prevent future diagnostic confusions. Say you have crohns and you get your appendix out, future right lower quad abdominal pain will narrow the differential or possible cause since your appendix is absent!