Thinking critically helps you analyze ideas and abstracts from more vantages and not just limit yourself to perceived reality unquestioningly without verifying authentic sources. Critical thinking requires to know about the subject from more angles than one and to consult as many sources as possible to a realistic picture of the said idea could be created. A critical thinker may be very good at journalism, science fiction and other such genres however being too critical can be a limitation for fantasy writers as critical thinkers always tend to be realistic and try to stay as close to a perceptible reality as possible.
By definition "critical thinking" is taking time and putting effort into considering the message you wish to convey to your audience. You begin with a premise and elaborate from that foundation. By doing so you create an objective, thought out presentation in your writing. It takes practice and the more you apply the principle of critical thinking it will begin to flow over into everyday situations. I'm not a "professional" writer by any stretch and I don't have to be to know how much more useful something I write - poetry, prose, essay, whatever - becomes for my audience when I've "critically thought" through my message. I want to engage them. I don't want to lose them! Good luck to you.