In an immediate sense, letterhead conveys a high degree of professionalism and shows the writer's legitimacy and establishes that they are a real individual acting as part of a larger organization. Letterhead also conveys an impression that the writer is important and an expert in their field and can increase the strength of the reader's association between the writer and their professional sphere.
Personalized letterhead gives a face and name to a corporate employee. This can help customers of a given business feel more connected to that business and develop a personal relationship with the personnel and thus the brand. A person is much more endearing to a customer than a simple form letter.
To further increase the impression of humanity, personal letterhead can include bio information, sales figures and other personal touches. These little pieces of information make the writer easier to identify with and outline them as someone who can be trusted and believed in.
Letterhead can incorporate other elements when ad space is in short supply. Detailed letterhead can include promotional offers like coupons and daily deals, adding value to each communication by weaving transactions right into the fabric of the conversation. The practice should be kept low-key but can be a valuable tool.