Men move forward only, which makes them one-directional and eventually exhausted. A king moves both ways, so it can attack, retreat, and control squares behind enemy lines. The difference is enormous.
How much is a king worth?
Roughly one and a half to two men, depending on how open the position is. In a crowded middlegame a king is worth less because it cannot manoeuvre. In an open endgame, a single king against two men often wins.
The practical consequence: trading two men to get a king is usually a good deal in the endgame and a poor one in the opening, when the board is too crowded for it to matter.
The back row dilemma
Your back row stops the opponent kinging, so keeping it intact is defensively valuable. But those four pieces are doing nothing else, and a player who never moves them is effectively playing with eight pieces against twelve.
The usual compromise is to hold two of the four — typically the second and fourth squares — which blocks the most common kinging routes while freeing the others to play. Holding all four into the endgame is a common beginner error that leads to being slowly squeezed.
The race to king
When both sides have a clear run, count the moves. If you can king first, you can often use that king to stop theirs, which converts a one-move head start into a decisive advantage.
A king defends the back row far better than a man does, because it can cover multiple squares from behind. One king patrolling your back rank can hold off two advancing men indefinitely.
Endgame with a material edge
Three kings against two is a win; two against one is usually a win; one against one is a draw. The technique with an extra king is to force the opponent onto the edge and then use your spare king to take away escape squares.
The key square in checkers endgames is the double corner — the two-square corner where a lone king can shuffle safely. Driving the enemy king away from it is the first step in most winning technique.
The double corner is the drawing zone
A lone king that reaches the double corner can often hold a draw against two. When you are winning, cut it off first; when you are losing, head there.
Opposition and tempo
As in chess, having the move can be a liability. In a blocked endgame the side forced to move must break their own formation. Counting tempo — how many moves each side has available before something must give — decides many otherwise equal positions.
The practical version: before entering a locked endgame, count how many free waiting moves you have. More waiting moves than your opponent means they run out first.
The first king wins games
If you take one thing from this: the player who gets the first king in a roughly equal position wins far more often than the material count suggests. Openings that delay your own kinging to prevent theirs are usually worth the investment.