Cassis Tegimenta
(helmet cover)
We know many things about the roman soldier's equipment while on campaign, but our current knowledge cannot definitely say that all Roman soldier's had an "official" list of objects to carry — as we know it, a "pack order." This kind of list would likely have varied or been changed by the commanders from time to time according to the Legion's situation.
Anyway, one of the common objects mentioned is the Cassis Tegimenta — a cover for the Roman helmet made of leather.
Also the literature has some mention of this cover.
We can only make conjectures about why the Romans were so careful with their equipment — in some cases some thoughts can be learned through the reenacting experience. We have an example fomr Vindonissa as an example.