In Bridge, When and Why Do People Sacrifice Overtricks to Be Sure of Making Game?
There are three popular methods of scoring, each of which treat overtricks with different value:

Rubber - Here the priority is making game (or multiple partscores) to finish the rubber.  Points earned for your contract go "below the line", and count towards winning the game and the rubber.  All other points (setting contracts, slam bonuses, hundred honors, etc) go "above the line", and do not count towards ending the rubber.  Since ending the rubber is worth 500 or 700 points depending on whether or not your opponents scored a game or not, you can see how sacrificing a 20-30 point bonus trick could be worthwhile.

Matchpoint Duplicate - This is the form of the game most often played at bridge clubs in partnerships.  Here, you score one point for each other partnership's score you beat when they played the same hands.  Since the scoring here is more absolute, there is value in winning an overtrick if you think the rest of the partnerships playing this hand would not.  For example, if you are in 4S, but feel that most people might be in 3NT making 4, you're going to want to try to make 5.  If you do, your 450 will beat everyone else's 430, and you'll get the maximum amount of points on that board.  Conversely, if you only make 4, you'll only score 420 on the hand, and your score will be worth nothing.  Since your 4S making 4 is worth nothing, you can see that 4S going down would also be worth nothing, so in this case it is worthwhile to risk the contract to get the overtrick.

International Matchpoints (or IMPs) - This is the form usually played in teams, where you and another partnership play against another foursome, and your results are compared directly.  Much like in rubber, in IMPs overtricks are not worth a whole lot. (see the scoring chart for the ugly details)  If you score an overtrick that your opponents do not, you get 1 IMP.  On the other hand, if you fail to make game while your opponents do, you lose 10-12 IMPS.  Here, securing the contract is priority #1.