War during the Age of Sail still had many of the features of a feudal host. Captains of ships, as well as commanders of regiments, were expected to recruit men. If one expected to eat, equip one’s command, or train above the rather stark allowances available either by way of the salary due one’s rank or the supply allocations one had to have access to cash. In an economy heavily dependent on gold specie, this could present problems to officers outside Britain.
Readers of naval fiction set during this period of time usually refers of banking. For instance, Dewey Lambdin’s character, Alan Lewrie, uses Coutts & Company as his banker. Coutts was one of the banks preferred by military officers and it operated through a system of letters of credit and a network of correspondent banks. It obviously was heavily reliant on the sense of honor of its clients as the lag time in transactions could take significant amounts of time.
This website gives a nice overview of how banking worked in the late 18th century.