Grounding a Club in Golf Meaning?

In golf, what does grounding a club mean? I don't get it.
It seams ridiculous that you can't touch the club to the ground in certain areas of the course.
I don't follow golf, but I saw Dustin Johnson was penalized for "Grounding a Club."
What is the purpose of this rule? How does it affect the game?
I saw highlights on SportsCenter and it showed it but still don't know what it means. All SportsCenter said was that you can't do it.

Can anyone explain the meaning of "Grounding a Club?"

Grounding the club meaning:
To place the clubface behind the ball on the ground at address. Grounding the club is prohibited in bunkers or when playing from any marked hazard.

The grounding rule essentially constitutes the penalty for hitting into one. Without the grounding rule, there might not be any difference in being just on the wrong side of the hazard line or just on the other side.
You have to be careful- obviously, D. Johnson's situation is one example.

The intent behind the rule of grounding your club is to prevent you from setting it down behind the ball before you hit it. It makes it much easier to gauge the shot when you can rest the club behind the ball. It is kind of like sizing an opponent up before you punch him in boxing by measuring the distance between you and your opponent with an extended arm. By not being able to ground your club you are suffering a penalty of sorts. It becomes a more difficult shot.