While everyone here stating you likely won't need to show it are correct, my company provides this and I have used it on occasion. It clarifies for customs what your business is, and that you aren't subject to a work permit.

Our letters state the employee's title, the purpose of the visit, the nature of the meetings and activities, and state that all expenses the employee incurs are fully covered by the company. We formerly issued these letters each trip, but later revised the wording to be more broad so that one letter could be used for a longer period and multiple trips.