3 ideas
Pieces placed in or commanding the central squares attack the greatest number of squares and can be redeployed to either flank, so the opening fight is fundamentally a contest for central control. Surrendering the center hands the opponent a permanent surplus of mobility that compounds across the whole game.
Tarrasch holds that a constricted position with limited piece movement is intrinsically losing, because the side with less room cannot coordinate defense or generate counterplay before a weakness is forced. The remedy is to free the position even at material cost rather than passively endure the squeeze.
Mobilizing every piece to an active square must precede any assault, since a premature attack with undeveloped forces gives the opponent time to bring more strength to the threatened point. The first principle of play is to get all pieces into the game quickly, never moving the same piece twice in the opening without cause.