Why pin knights early in the opening? I am a beginner amateur player, and I have been thinking about openings. On lots of occasions the opponent pins the white knight on Nc3 or Nf3 (as black) or the black knight on Nc6/Nf6. What is the main motivation for doing that? I am partial to bishops, and it seems even opening theory suggests that it is okay to get rid of one or even two bishops early in the opening at the cost of your opponents pawn structure? It seems very counter-intuitive to me. Can someone give examples of games/tactics where the pinned knight gives a big advantage for the opponent?
One of the key principles of opening play is that control of the center is vitally important. If one of the players has complete control of the center then they can much more easily launch an attack and it is much more difficult for the other player to defend.

A white knight on c3 supports/attacks e4 and d5.
A white knight on f3 supports/attacks e5 and d4.   

Those 4 squares define the center of the board. A similar story is true for black knights on f6 (controls e4 and d5) and c6 (controls e4 and d5).

A pinned knight doesn't support or attack any squares. It is pinned. It can't move. It no longer plays any role in the fight for the center although it can come to life again if it is unpinned.

Hence the normal progression in the opening of moving a center pawn 2 squares to control one of the center squares, then move one or more knights out to also control center squares, then move bishops out to either control center squares or to pin opposition knights which has the same effect.