not this argument again please
Technically, it's only worth 3 points because a win will only give you 3 points. But in terms of the title race, it's worth 6 points. The caveat is that you have to accept a looser definition of the word 'points'. On any given matchday, there is a maximum of 6 points available to Barca and Madrid combined. For the title race, the net points you gain is not just dependent on your performance but also on your main rival's performance. On most matchdays, your actual game is only worth 3 points, because your result does not affect the result of your main rival's. But in a Clasico, your result directly affects the result of your main rival's. Which is why it's worth 6 points. Because it combines the results of two games into one.
Think of it this way. Normally, all the games are worth 3 points on the table. But even if you get those 3 points, you don't actually change your position with respect to your rival because they also won. Your points total is +0 with respect to your rival. But in a Clasico, if you win, you actually change your position with respect to your rival. Your points total goes up by +3 with respect to them. Which means that, in effect, the game is worth 6 points.