Why causality is important to a plot?
What is causality?
In fiction causality is a chain reaction that occurs between events in your plot. It makes one event cause another.
A example of causality is when a character sets out to seek revenge against someone that has done them wrong. If this character had not been wronged they would have no need to seek revenge.
What happens when causality is omitted from a plot?
If there is no causality in your plot then your story is simply a stream of unrelated events that occur one after the other.
An example of this is:
A man walks down a street, he sees a cat, there is a car crash, he goes home, the end.
Without causality in a plot it is merely a stream of consciousness. There will be no point to the plot.
Your characters do not grow and learn. There are no truths realised, and no change has occurred.
What's the purpose of a plot?
Remember a plot is a complete cycle. In very rough terms there should be a beginning, a middle and an ending. This cycle should be satisfying upon completion such as the answering of questions, the realisation of truth. Maybe your readers will be given the chance to learn something about the world they live in, something new they can make sense of such as human nature.