Golfclubs requiring a minimum required handicap are often renown golf courses or really challenging golf courses. By providing this barrier the golf clubs ensure that only capable golfers will play on their courses. It also ensures that some golf flights with less experienced golfers will not hold up others.
A handicap is used to determine on which holes a player (or team) is granted extra strokes. These are then used to calculate a "net" score from the number of strokes actually played ("gross" score).