They were deeply private, and unvetted by the authorities (as opposed to many letters home), so to read them is to be given a bird’s-eye view into the diarist’s innermost thoughts.
Unsurprisingly, they often offered a bleak assessment of life at the front. Take this entry from Harry Drinkwater of the Birmingham Pals: “This is not war, it’s a slaughter. No man, however brave, can advance against a sheet of bullets… it appears to me that the side who will win will be the one who can supply the last man.”
Other diarists preferred to soften the grim reality with dark humour. Captain Alexander Stuart, for example, described his annoyance at having to stop smoking to shoot a German who had got into his trench.
Thousands of soldiers kept diarists during the war, of course, and thanks to the wonders of technology, more and more are becoming available to the general public on the internet.