King

Your King is your most valuable piece. When you start out your opening, you want to develop tempo for your Knights,Bishops,Pawns. After you open your pieces, CASTLE!!!!!!!!!!! Over the series of games that I played, when I didn't castle I lost most because of the King wasn't protected.Castling also makes your rooks get into the game and attack  the middle.

Good by protects the king with your pieces attacking Black with also preventing most attacks from castling.

Here, you don't have to castle because the main threats are out of the game.

This is bad when you don't catle because your King can attack more and is weaker when not castled with many threats on the board.

Later in the king the value goes from 1point to 4points because of the pieces taken out of the game and becomes a major piece for pawn promotion.

Castling leads to better protection and more tempo for your pieces.