Why distillation? Why not adsorbption or leaching ?

In distillation the new phase generated is different from the original by phase, or heat content only. This heat can be removed or added by easy operations. But in case of adsorption or leaching the a foreign substance is introduced to separate the phases. The new phase generated using these processes is a new solution which in turn may be separated using other separation methods unless the new solution is directly useful. This makes the distillation process to more economical.

Distillation process depends on the relative volatilities of the components. If the difference is too low separation is difficult and it makes the process as more expensive.