All the great evils which men cause to each other because of certain intentions, desires, opinions, or religious principles, are likewise due to non-… - Moshe ben Maimon

This is self-evident.

The reason of a commandment, whether positive or negative, is clear, and its usefulness evident, if it directly tends to remove injustice, or to teach good conduct that furthers the well-being of society, or to impart a truth which ought to be believed either on its own merit or as being indispensable for facilitating the removal of injustice or the teaching of good morals. There is no occasion to ask for the object of such commandments; for no one can, e.g., be in doubt as to the reason why we have been commanded to believe that God is one; why we are forbidden to murder, steal, and to take vengeance, or to retaliate, or why we are commanded to love one another. But there are precepts concerning which people are in doubt, and of divided opinions, some believing they are mere commands, and serve no purpose whatever, whilst others believe that they serve a certain purpose, which, however is unknown to man. Such are those precepts which in their literal meaning do not seem to further any of the three above-named results: to impart some truth, to teach some moral, or to remove injustice. They do not seem to have any influence upon the well-being of the soul by imparting any truth, or upon the well-being of the body by suggesting such ways and rules as are useful in the government of a state, or in the management of a household. ...I will show that all these and similar laws must have some bearing upon one of the following three things, viz., the regulation of our opinions, or the improvement of our social relations, which implies two things, the removal of injustice, and the teaching of good morals.

The question, "What is the purpose thereof?" cannot be asked about anything which is not the product of an agent; therefore we cannot ask what is the purpose of the existence of God.