It makes difficult issues, like disciplining, easier to deal with, even if you don't always see eye to eye. Kids are so perceptive, they'll take any opportunity to play one parent against another. But when you feel close to your spouse, you're less likely to disagree with him in front of the kids. Close couples present a united front to their children. Even though they may have different discipline styles, they support each other's decisions and don't question them in front of the children.
Children feel more secure, because they know what to expect.
Parenting is easier because you and your spouse are united. If children consistently see their parents' needs taking second place to their own, they can become very selfish, and parents will have no one to blame but themselves. A strong marriage can benefit your family now, but it will also hep your children down the road. A parent who pays attention to his or her spouse's needs will model a positive relationship which pays off in adulthood. As much as people might say they're not going to be like their parents, you are to some degree.