Some of the characters in this play provide a powerful demonstration of the dangers of lacking common sense and reason! Moliere seems to be arguing for the need to have moderation and common sense in all things. Consider how Tartuffe and Orgon are both presented as men of excess and obsession. Tartuffe is of course a glutton, and Orgon becomes obsessed with Tartuffe to such an extent that he completely loses his sense of reason and nearly ruins his own family as a result. Even when the truth emerges and Orgon finds out that Tartuffe is actually something of a fake, Orgon reacts in a typically extreme way, saying that he will "never" trust another man and will implacably oppose himself against those who say they are virtuous.
In comparison to these two characters, that provide ample learning opportunities concerning the need for moderation and common sense, Cleante provides the voice of reason in the play, counseling Orgon about the need for moderation and advising him to chagne his character. Note what he says to Orgon:
You never are content with moderation... you fly back and forth between extremes.
It is Cleante that tells Orgon what conclusions he should make about the world following his friendship with Tartuffe. He must not oppose himself to all men who appear good, but adopt moderation in his own character, especially in the areas of judgement and behaviour, so that he can act less compulsively. Cleante is thus the vehicle used by Moliere to announce one of the key messages of the play: moderation and common sense are much better than obsession or the lack of reason.