In Baduk, black and white alternate turns, each placing one stone on the board. The board has a limited number, 361, of intersections, so the first player gets an advantage. To have a fair game, the first player, black, must offer something to the second player, white, to compensate for this advantage. Dum [komi in Japanese] is a fixed number of points given to white, to be added to the final score.
  The usual practice before the adoption of Dum was that the stronger player gets white and the weaker player gets black.

But changes in the Baduk community (for example, the birth of Japanese Baduk Association) brought on rapid growth, and this narrowed down the gaps between players. This made the adoption of Dum essential to have accurate contests between players.

Dum was first introduced in 1938 in the first competition for the title of Honinbo after Shusai, the last hereditary Honinbo, ceded the name to the Japanese Baduk Association. The value of Dum used in this competition was 4 points.
Later, this value was increased to 4 and a half points.
  The reason for this change, interestingly enough, was that two players in one of the matches agreed beforehand to play a tie game, each scoring the same number of points, to make more money (they were paid by the number of games they played).
  If they had stopped at one game, they probably would not have been caught. But after they had another tie in the second game, the host made an investigation and uncovered the prearranged agreement, and made a strong protest to the Japanese Baduk Association. The Japanese Baduk Association scolded the players strongly, and they apologized, and started the re-match. Laughably, the game once again ended in a tie. The half point was added to Dum from then on to ensure that a game could not end in a tie.

-- by Jung Yong-jin, Editor of 'Baduk' magazine