Here's the game:
lichess.org/jVrszLO0/white#14I'm guessing it's better since you get a lot of space as white which allows for your pieces to freely develop to nice squares while black is struggling. But if someone can explain it to me in a better way I'd really appreciate it (or correct me if I'm wrong).
EDIT: It should be "8th move for white" in the title, sorry!
You're also damaging Black's pawn structure, creating doubled pawns and possible an isolated h-pawn.
Also, Black's Knight is fairly powerful, while your Bishop is kind of sidelined and not "in the game," so this means that you'd be trading a less powerful piece for a more powerful piece. Especially given the Knight's control of the centre. So yes, in general this trade is favourable for White.
Final thought: if Black captures with the e-pawn this creates a nice outpost on d5 for your Knight to occupy.
It's a good thing you're going back and trying to understand your game. When I do this I like to try and figure out my first mistake in the opening so that I can perhaps get a little further the next time I face these or similar ideas. In your case you're practically winning the game after 4...b6, so I think you're actually asking the wrong question. Your first mistake is 6. e3. Not a bad move, but not as precise as you can be. So I suggest you try and figure out why dxc5 is the better move.
As for your question, again, it's the wrong one. ;)
If 8. Bxf6 is the best move, then surely it's even better on move 7. It's actually quite instructive. My first hunch as a d4 player is to try and build a big center, but it's not worth it at the cost of weakening the dark squares after 7. Bxf6 exf6 8. e4 with perhaps 9. d5 to follow. But you don't have to do it that way. You have Nh5. If Bh7 black is very inconvenienced by Bd3 -- a common idea in the advance caro-kann. If black allows white to take on h7 then having to play Rxh7 disrupts development and coordination. And if Bxd3 black helps white develop the queen. After 8... Bd7, the best move according to the engine, then white dominates the light squares. There's a cool line that goes 9. Bd3 Nc6 10. Ng6! because if 10.. fxg6 white wins with Bxg6+ and Nd5+ and Qg4+ to follow.
General concepts can help you (Bishops are good, let's use them to harass knights, but not capture them! Knights on the rim are dim!) but you can't improve your chess if you aren't willing to try and find concrete reasons to break those mini-rules. And there's concrete reasons why Bxf6 is best -- best articulated by the fact that black's king is stuck in the center and development of the remaining pieces is a problem because the light squared bishop is horribly misplaced and white can gain time attacking it.
That makes so much sense! Capturing the knight there also enables me to get black's light-squared bishop out of the game and get more control over the lights squares in the middle and working off of what you said earlier about my question being the wrong one (which I agree with now), that would mean that the evaluation is as high as it is for that move because of black's b6 giving you extra advantage in the form of the rook being exposed along the light-squared diagonal.
As for the last paragraph, that's something that I recently realized and it improved my game massively. Instead of looking at moves as good or bad based off of general concepts like developing early, not losing tempo, etc. I started to look at the game as "Look, the goal of the game is to checkmate the opponent's king first so make sure that every move you make gets you closer to that", which allowed me to come up with better ideas and instead of thinking about the game from a very high-level (as in not close to the core of what chess is) it allows me to concentrate on what's actually important in the game. The best example of that in this game is the move 10. g4, my brain was trying to convince me that it's a bad move since you don't want to be weakening your king-side if that's where you plan on castling, but because I used the mindset I described above I actually did play the move as I didn't see a way in which black can exploit the weaknesses I'm making because of the attack I get in return. Wow, I'm really going on a rant here lmao.
All in all, thank you so much for the explanations!
after bxf6 black's structure is very bad. yes you give the bishop for a knight but in the long run, especially on an endgame, black will be worse due to that