Rashi comments that the plural form is used in the above verse, even though the angels did not assist God in creating man. He goes on to explain that even though in using the plural the Torah leaves itself open to heretical claims that many gods participated in the creation of man, nevertheless God wanted to teach a lesson in humility and derech eretz (respectful behaviour).
God wanted us all to understand that great people should take advice from those that are lower than them. Had the verse said “I shall make man”, then we would have not been able to learn this.
One could question as to why God chose this precise moment to teach this lesson. After all, heretics point to this verse to back up their claims of a lack of belief in a solitary God.
Surely God could have chosen another opportunity to teach lessons of humility and good conduct at another less contentious point in the Torah?
Rabbi Noson Tzvi Finkel, the Alter of Slobodka (1849-1927), answers that good middot (character traits) and derech eretz are a prerequisite to belief in God.
God was willing to endanger the faith of those reading this verse to teach them the importance of derech eretz, because a belief in God without derech eretz is not enduring. One who does not truly appreciate the importance of acting humbly or asking advice from those less knowledgeable than themselves cannot properly believe in God.
Our rabbis teach, “Without derech eretz there is no Torah” (Ethics of the Fathers 3:21). Without positive character traits, then all of our Torah endeavours will be lacking. Being a good Jew is not just about Torah study and mitzvah observance, it is also about being a mensch.