When a group is captured it is replaced by red stones (rather than being removed). A red stone behaves just like an empty position, except that no-one can play a stone there.
At the end of the game, a player's score is the number of stones of their color on the board plus the number of empty positions and red stones surrounded by their stones.
A game is considered good if there are few or no red stones. (In a round robin tournament, the winner is the player with the highest total score. Thus red stones detract from both players.)
Why?
The game always ends, board configurations can't be repeated. (there's no need for a no repeats rule)
Capturing a stone gives the capturer an unkillable eye. You really don't want your stones to be captured. (the usual Go rules are also rigged to discourage capturing, but in a way that makes working out the final score complex)
It's a metaphor... don't try to invade a country twice. :-P