Let's revise the requirements needed to win in Japanese mahjong:
4 mentsu + 1 jantou + 1 yaku
You need 4 mentsu, while you only need 1 toitsu, and the formation of toitsu is easy. Often we do not need a lot of toitsu, unless we're aiming for a chitoitsu.
The extra toitsu, may become a mentsu, a toitsu only need to draw a third tile to become koutsu. However the chance of successfully turning into a mentsu is not high, as there are only 2 effective tiles, this is lesser than the 4 effective tiles of penchan. (Reader note: you might think of calling pon on discards and gambling on a toitoi. However since we're discussing about tile efficiency, the reader should put aside this discussion. The theory related will be explained during the yaku theory in the future.)
Hence, once the number of toitsu exceed what you need, the extra toitsu will become very inefficient. A common mistake between beginners, is keeping too many toitsu, thus slowing down their hand and being unaware of it.
The text below explains the basic principle of handling toitsu.
You should not determine the jantou too early
The need for jantou is low and it is easy to form. When the shanten number is high, and there is only a single pair in the hand, determining the jantou too early will put you at a disadvantage.
There are many people who would determine the 4 pin as the jantou too early and discard the 6 pin. However I am unable to accept this kind of thinking. The correct way of thinking is treating 2446 as two kanchan. Discarding 1 pin here is the correct answer.
The 23 wan here along with 34567 has the highest chance of becoming 3 shuntsu, but where would the fourth mentsu come from? If you discard 6 pin, the wait for the fourth mentsu would be 79 wan and 12 pin, but they are bad shapes, and have a low chance of good evolution. Succeeding is not going to be easy.
Discarding 1 pin, and keeping the chance of forming a mentsu with 5 pin is very important.
Even if on the side of manzu, 79 wan turns into 78 wan
If it's still at the early stages, discarding 1 pin here remains a very strong discard. Even though 2378 wan is a good shape, this sort of tiles do not guarantee a 100% chance of being added into the hand. In truth, drawing a kanchan tile is a common sight, an extra mentsu as backup is never disadvantageous.
Someone might ask, is it disadvantageous to draw 3 pin after discarding 1 pin?
It's a little disadvantageous, but it's not severe. The loss here is only a jantou. Drawing 2378 wan, 14 pin, 3467 wan will replace the lost jantou. The situation is better than discarding 6 pin and drawing 5 pin.