At the National Archives, London there is a useful class of papers that is not easy to use because it is indexed only by name, sometimes a ladies name, with no unit. Until you inspect the contents of a file you have no idea if the file relates to your subject, if you are paying a researcher this could be expensive.
The papers are Pension Papers, class PIN 71. They contain correspondence about the recipient's or widow's pension, most time there is a summary of the military career which can supplement service papers. In cases of a medical discharge or death in service there are details of the medical reasons behind the discharge or death, in the case of a wound vital details can revealed about the circumstance in which the man was wounded.
However, there is a clue to be found that can help determine if you should look in PIN 71. On the service papers found in class WO 97 (discharge prior to 1914) there is sometimes written on the first page a code ending "FW/M" - "FW" stands for "former wars".
This says "Go check PIN71" on the National Archives catalogue - Discovery.
Pension correspondence is not the most exciting, but as it concerns money there are useful details to flesh out the "man behind the medal" story; home and work place addresses, details on next of kin - wives and children and if you are lucky surprises. One recent PIN 71 file I read for a soldier I was researching showed that, while a soldier, he had two deductions made against his pay by the civil courts in respect of illegitimate children; gold dust for family historians!
If you are researching a soldier who died on active service there won't be service papers so no "/FW/M" clue. However, if you know his next of kin's name, especially if he was married, then it is worth checking PIN 71 using the next of kin name. I recently read the file for the widow, Clara, of Pte 2743 Walter Gibbard 4th Hussars. She received 5 shillings a week, in April 1918 a war bonus of 5 shillings was paid and in 1919 the pension for men killed in wars before World War 1 was raised to the level of that for men killed in World War 1.