"What 'good' do you derive from letting someone over into your lane so they don't miss their turn?"
1) They don't in desperation try to get over and cause an accident, possibly involving you.
2) They feel some gratitude and perhaps do something considerate for someone else so the world is a happier place.
3) They don't become angry and engage in some stupid act of road rage.
4) The above when multiplied by a number of considerate acts probably reduces accident rates, and perhaps causes savings on your car insurance.