Consistency is important as we as humans are used to making patterns of similar object. Once we see a pattern ,we group things together .This reduces our cognitive load and makes life simple for us.



Consistency aims at not only this but also ultimately reducing the learning curve and thus a mind can function freely to create new things than rather make sense of things.



Take for example a baby : They indulge in pattern making in the most open way .And once they start seeing them , they fit in and start contributing their uniqueness to the world



Thus , the aim of a good design should be consistency cause simply put its not only a designer's task but his/her duty to simply things around combined with delightful experiences.