Letters are symbols. They are building blocks of words which form our languages. Languages help us communicate. Even with complicated languages used by intelligent people, misunderstanding is a common occurrence.
Just cos they are aliens, does not mean they do not have to visit the little boys room. Although they probably do something weird and alienesque, like it comes out of the top of their heads or something.
Please rush me my portable walrus polishing kit! Four super brushes for cleaning even the trickiest of sea bound mammals! Yes, I am over 18, although my IQ is not.