Descartes' dream argument states that human senses are flawed, and that we cannot fully understand reality by relying solely on them. To show this he provides the example of dreams. Although dreams are not real, our senses often believe them to be.
The fact that humans are not always able to distinguish reality clearly shows that we should not rely on instant justice. In order for us to access instant justice, we would have to know for a fact what the circumstances surrounding a crime are, without any further analysis. This is normally consider illegal. Instead, most societies access justice through a long process that includes arrests, trials and testimonies. This is intended to offer a clearer and less fallible understanding of reality, and therefore, a more just judgement.