A positional Piece sacrifice to keep the Initiative!
Please look at the following position from the game Petrosjan - Pachmann
White has here a slight advantage in development of pieces and therefore initiative.
The main problem for black seems to be the development his knight. White controls with Qg4 and Ne5 the square d7
So black had played now 16. ...f6 and tries to remove this annoying outpost knight from e5. Then he would be able play Nd7 and complete his own development.
White is here at a crucial point! He could perhaps loose the initiative by playing too "peaceful" now .
So he decided to go for a sacrifice to maintain the initiative