Deists often stress the importance of reason when it comes to arriving at any truths but especially metaphysical truths. If a claim or proposition lacks proper justification, it ought not to be accepted. While this may seem obvious, there are plenty of doctrines across the world that flout reason as it is inconvenient to their narrative.
Reason is the ability to come to valid conclusions based on the use of logic. This entails starting with premises and using deduction to come to a conclusion based on them. Reason constitutes our ability to understand ourselves, and the world around us. Disregarding reason in favor of more dubious methods of inference can have terrible consequences.
Reason prevents belief in irrational or unjustifiable assumptions. For example, if I were told a square circle exists, I would be highly skeptical of it's existence due to it's logical impossibility. The concept of a circle and the concept of a square are mutually exclusive and cannot exist as one shape at the same time. Another example would be the existence truthful lies. By definition, a statement can either be true or false. It may be possible to have propositions that may contain a true and false statement, but an individual statement cannot be either simultaneously.
We cannot understand truth without reason. If we allow ourselves to believe in logical contradictions, we put the concept of truth at risk. Reason is important for evaluating one's actions. Our beliefs govern our conduct in the world and it is instrumentally better to have our actions based on true beliefs rather than false beliefs. The Deist mission of proving God has been made harder by other attempts and arguments that often rely on faith rather than reason. However, we are convinced that the proper use of reason indicates the existence of God. There is much debate over which justifications are valid, but it is at the very least important that we know how to methodologically approach knowledge of the world we live in.