The reason for this is that the duration of the separation between the two countries has an impact on how different they have become.
If you are writing an essay about whether the two Koreas can be reunited successfully, you will naturally want to compare them to East and West Germany. You might conclude that, if East and West Germany could successfully reunite then it is clear that North and South Korea can reunite. After all, the two cases are very similar in that both are countries that were split apart by outside forces at the beginning of the Cold War.
However, it would be a mistake to simply say “if Germany can do it, Korea can too.” One reason for this is that Korea has now been split about 50% longer than Germany was. Germany was split for about 45 years and Korea has remained split for the 22+ years since Germany reunified in 1990. This is important because it allows the two Koreas much more time to grow apart economically, socially, and politically. The more the two Koreas are similar to one another, the easier reunification would be. The fact that they have remained split for so much longer than the two Germanys might mean they will have grown much farther apart.
This is an important factor, then, because it prevents us from simply saying “since Germany successfully reunified, Korea can do the same.” Instead, it forces us to think more deeply and to consider whether the case of Korea is really similar to that of Germany.