Obtaining Reasoning from the Engine

I just miscalculated something and lost a pawn. However, white can still try to fight for some advantage. The leading computer line says Nd3, which looks like, what? What does that do? 

It's not like c5, which clearly attacks the bishop (Although why you would want to attack the bishop is beyond me)

It's not like f3, which was played (and a blunder), which attempts to immediately force the situation with something like f3, Ng5, h4, etc. 

So why Nd3? 

The first step is to play the move on the board. Like please do it, trust me, I will be scrolling on reddit and these people with diamond membership be posting like "bro i dont understand this puzzle/move/line the engine suggests? Why do you play this move?" and you can clearly see they hadn't even explored the line that the engine suggests. Which is like if you were in math class, and you didn't know how to do a problem, when you haven't even checked the textbook examples yet. Like, at least try?

We played Nd3 on the board, but it's still not clear why you'd want it. The computer recommends Be7, but we don't know why. We can try to figure out the reasoning behind Nd3 by making black play a waiting move. I chose to make black play a6 (image above). 

Looking at the lines again, you'll see that it suggests something interesting...

c5 is played, but I felt like Be7 would be more natural instead of Bb8 or Bc7.

After Be7, the computer suggests h4! And then everything is clear: Where is black's e4 knight going?

White's going to play Re2 or Re1 or something and easily win the knight. Black will usually have to sacrifice it on g3, but there's no follow up... But if the black bishop was on c7 or b8, it might work.

But going further than that won't really matter that much; it's enough to know the purpose of c5 followed by h4. to take away squares from the e4 knight.

But why do we want to play Nd3? Why can't we just play c5, straight up?

Easy. If we don't know, play the engine moves.

Here, after c5 and Bc7 (we now know why Bc7 is played over Be7), we can see now that if h4 is played, black can play Nd2 since his queen protects it. Nd3 is played to block the queen's line of sight. It all makes sense. 

But now there's one more thing. Why play Nd3 first, and what's worse about playing Nd3 after c5?

Well there's a few things. First of all, after c5 and Nd3, black will know that his knight is not safe, and will likely just move it out. Wheras playing Nd3 first masks your intentions, making it less likely to spot. 

Think of it as if you were playing black; If white played c5 and Nd3, you're likely to figure out that he wants to trap the knight compared to playing Nd3 first, which just looks like it gets the piece developed. And Nd3 keeps white's pawns flexible, which is usually desirable because pawns can't move backwards.

Is my explanation correct? I think so. Maybe it's not the way the engine thought, but it makes sense, and I learned something new. And what more can you really ask of the engine?

Now let's scroll all the way to the top. White just played Nd3, and the top engine line suggests Be7. "We don't know why", I wrote previously. 

So why would black play Be7 after Nd3? 

Because it gives the e4 knight a square to come back to. Now the knight can go back to g5 or d6; so if c5 is played, black can play Ng5. And why can't black just play Ng5 instead of Be7 if he's so worried? 

Well, let me introduce you to the second concept about the using the engine: Black CAN play Ng5. It's the second top engine move. 

SO LONG!