What is the reason for the automatic fruit and nut dispenser on some bread makers?

Fruit and nuts can potentially soak up some of the liquid. The liquid is then removed from the hydration of the bread during rising/proving, so you might end up with a mix that is too dry to form a loaf or that is unable to be kneaded properly or to rise properly.

Adding the fruit and nuts after some mixing and kneading means that there is much less available liquid for them to soak up, as the liquid is bound in with the flour already.

Edited to add: In many instances bread machines are on a timer, so that the bread is ready when the person wants it to be. If fruits were added into the liquid before a timer-controlled loaf, then they may well not only absorb a portion of the water, they may over-swell and become so soft that they disintegrate into the bread during kneading. This is generally not the desired end-point of having fruit in breads, where most recipes have the fruit as visible pieces.