Certainty has very high survival value in lots of different domains in life. That is why people feel more drawn to someone who provides definite answers to life's big questions than another person who qualifies their answer and appears to be long-winded.
Advertisers make use of certainty all the time: E.g., Our product is the best, etc.
People who predict the future or who say what they believe is going to happen in the future will get no where fast if they sound uncertain. Hence, religions that do best (i.e., in terms of recruitment) usually make very definite statements about what the future holds and what we need to do to enjoy that future.
Two well known people who have spoken with crystal-clear certainty are fundamentalist preacher Billy Graham and athiest Richard Dawkins. Certainty is definitely a human need!