>>294254114,5,6, being in the 'center' of a suit, are versatile because not only can they combine with a tile on either side of them, unlike terminal tiles, they also allow an easy hand called 'tanyao', which can be open or closed - that is, formed by taking another player's discards midway through the game, or without. The rule of this hand is that it cannot contain any terminals.
Discarding these tiles indicates you do not need them - which implies you have better melds already in your hand that are compelling enough for you to discard this versatility. So assuming you are not a complete newbie, that makes your hand look very dangerous, because those other hypothetical versions of your hand are assuredly much higher scoring.
Note that a discard in riichi mahjong carries more weight because you cannot win on a tile you discarded. That means that if you discard a 6 but later need a 6 to win, you cannot - a state known as 'furiten'. Note that you cannot win off the OTHER end either - for example, if you were holding 4,5,6, and discarded the 6, you cannot win later with 3,4,5. This concept underpins all discard-related strategy and defensive measures in riichi mahjong.
