In the Fellowship movie, Saruman gives a long speech to the (newly created) Uruk-hai chief about how the Orcs first came into being and then proceeds to ask him about his loyalty. Why does he do this?
Aside from the necessity of informing the audience, I think there are some real in-story reasons that we can attribute to this.
Orcs in general did not have much in the way of culture, particularly these Orcs created as they were by Saruman, and so how they perceive themselves, their background, and their place in the world has a lot to do with how they will fight for Saruman.
Saruman is moving from a position of actually being very wise and being exalted by those around him to becoming nothing more than a two-bit hack who can't even maintain his position with his own flunkies. And so, within the narrative of the story, this sort of grandiloquent soliloquy delivered to the leader of a group of marauders fits both his background and the general direction of the story.
Also, just because it's Tolkien and that stuff isn't going to exposit itself.