How many times have you received an official letter bearing a letterhead? If those letters are coming from companies, business, organizations, and individuals, the probability is that all of the letters bear such.
These are part and parcel of official correspondences. If a letter, supposedly coming from a big-shot company does not bear one, make room for doubt. That may be coming from elsewhere. No company sends out official correspondences without using such.
There are different advantages, aside from the practical reason that it easily shows from which company or organization the letter comes from. It foster brand recognition or brand image as well as sets a corporate presence. It means that