By putting different sets of reactions into different organelles they can be run under optimal conditions. The lysosomes are a good example of this principle. They are filled with digestive enzymes that work best around pH 5. If the enzymes were in the cytosol they would not work well because the pH there is around 7. Lysosomes are adjusted to pH 5 to optimize the digestive enzyme activity.
Organelles also give protection to vital structures.
Putting the DNA into a separate container protects it from attack by many of the chemical reactants found in the cell. The nucleus also stockpiles repair enzymes at high concentration to correct the defects that do occur.