Although I am not familiar with the RN version of the document, it sounds as though it is similar to the Royal Australian Navy service record I have worked with.
My strategy has been that it is relatively easy to find rich references listing where particular ships operated and what they did (both in print and on-line). You know from the summary when your ancestor was on board, so you can cross reference to find the operations in which he was engaged.
One of my projects is to map the locations of each pair of ships when my father transferred from one to another. You can quickly build up the skeleton of a timeline with map that documents his service.
A simple web search for the ship's name (with relevant years) can sometimes find the published reminiscences of men who may have been his shipmates. Or at least who tell stories that will be similar to his experiences.
One caution is that the Navy has a habit of reusing names for vessels. So be a good genealogist and always check its "date of birth" (either keel-laying, launch date, or commissioning).
Some specifics on Henry Griffiths
Your document shows that young Henry committed to a 12 year term on his 18th birthday, but that he was in the Navy before that because he was posted to HMS Vivid (a training unit in barracks at Devonport) in July 1914 as a "Boy". So he joined up a fortnight before the war began.
His first ship HMS Powerful refers to one of two training units (based at Devonport) but aboard either HMS Andromeda (a Diadem-class of protected cruiser launched on 5 January 1898. converted to a training ship and renamed Powerful II on 23 September 1913) or HMS Caroline (a Satellite-class composite screw corvette launched in 1882, reassigned to harbour service in 1897 and then a training ship renamed Powerful III in 1913).
Several different vessels were used in Devonport for training purposes during WWI and rebadged Impregnable with a number while in that role. Some of them have truly amazing histories. Can you imagine your ancestor on HMS Howe (Impregnable I), a three-deck ship of the line on a keel laid down in 1860?
You have an absolute treasure trove here!