"All men are created equal" is a fiction but very often believing a fiction is better than not doing so.
After all the fact that this is the core belief of the USA is better than the opposite that people are created unequal which opens to door to not nice things. You do want to treat people according to their actions.

And this is  a general principle. Sometimes believing in  a fiction can be helpful since it will help protect you from other fictions that might be much worse.