So, when you plop the capital down on the first turn on a luxury you start out with 11 points on Chieftain (9 tiles +2 for the happy person), and 66 points on Diety (9X6=54 6X2= 12 54+12=66). It's usually easier to add another city (9 more points on chieftain, 54 points on Diety) than to try and get 9 people happy (that had been content, or from unhappy to content) to match that. Or, to get a city's border expanded (12 pts. on chieftain, 72 pts. on Diety). But if you have hundreds of cities, then getting one more luxury could mean hundreds or thousands of points.