Question

why crystallisation method is considered better than the simple distillation method

Answer 1

because it's easy than distillation method and kills more microbes.

Answer 2

Crystallization is a process separating a pure solid from a solution in its crystal form. "Crystallization is far better than evaporation" because some solids decompose during evaporation, or some such as sugar can be charred to dryness when heated.

Some impurities in the solution may remain dissolved even after filtration that contaminates the solid after evaporation. In comparison with any other methods, in crystallization we can get the purest form of any given solution.