Scutisque Tegimenta (shield cover)
We know many things about the roman soldier's equipment while on campaign, but our current knowledge cannot definitely say that 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 scutisque tegimenta — a cover for the Roman shield made of leather.
Also the literature has some mention of this cover.
"The time was so short and the enemies so fast that the Romans didn't succeeded to prepare the standards and also to put on the head the helmet and to uncover their shields."
We can only make conjectures about why the Romans were so careful with their shields — in some cases some thoughts can be learned through the reenacting experience.
Also, we must remember that the scutum, together with the standards, was a key object that helped identify each soldier as member of a certain group (cohors or centuriae), this means that the scutum would be always clean and with bright colours. By not using the cover, in fact, the shield would be exposed not only to dust, obviously, but also to mud, rain and, overall, the sun. Sun can be the worst enemy of the shield and can cause the bright colors to fade and the leather/wood to dry. Rain and humidity could cause the wood core of the scutum to swell and delaminate — something to be avoided.