I would agree with Nane's opinion of this as just a basic answer. Preventing someone else from getting something can definitely be worth something to you. A good player should recognize that stopping your opponent from getting 5 points is the same as you getting 5 points, but if another move gives you 2, you net 3 from blocking. (In 2 player. In multiplayer that gets more complicated.) But I've never seen the point of blocking just for the sake of blocking, or even doing if if your opponent simply cannot win and you don't need to.
But addressing some of what Jest said. I think that Scenario 1 depends on the situation. (So it depends on the situation, which still depends on the situation.
) If that was your last move, or near the end when you can't really do anything else, then I'd agree. But otherwise there could be a point in choosing to hold back Player A or B, and purposely make it one of the two. Which will depend on what those moves are that you can prevent and what else they can do, what the chances are they can do it, and what they may be likely to do on subsequent turns. I always struggle with what to do in multiplayer. If it's a last turn and it just comes down to math, that's a simple decision.
But I also think blocking can be long-term in many cases. Or maybe I've just played too much Carcassonne. (Not here) It can be useful to block something right from the start. (What's even nicer is that you can often block and do something for yourself at the same time.) But there are other games where spending too much time blocking someone else ends up being to the detriment of what you are trying to accomplish. (Simple example - Ticket to Ride.) You do need to consider the long-term benefit or cost of doing so.
You can also complicate this by whether you are just trying to win, if it's your best buddy and the two of you have fun dumping on each other, whether you like to see how high a score you can get (win or lose), etc.