Bishop takes knight

In some openings and games I see a bishop pinning a knight to the king or queen and bishop takes. Outside of doubling pawns I was wondering what the theory is here since I've always heard that a bishop pair is more favorable and I usually don't see an immediate or even long term advantage of doing so. So whats up? Lol

what openings do u use when play chess ?

I'm playing e4 now. I usually play Italian but it depends on what black's defence is

The chapter on Pin in Nimzowitsch's "My System" is a good read. I also reinforce it by watching the YouTube videos so it's burned into my brain.

YouTube "Nimzowitsch My System" and skip through the videos until you find the pin videos. I've read the book more than the videos, I'm on video 11 on YouTube so Pin will be a few after that.

Edit: A lot of the examples are based on the Italian and four Knight's game so this should be fine for you.

In some positions, that if black exchanges his WSB for whites night on f3 it accomplishes several things, Black trades his weak bishop for a very good White Knight and also causes white to either capture with his Bishop and misplacing it. This can help Black in 2 ways, Whites attacking chances on the kingside are weakend and Black can gain time.