Obviously the practical side is to protect SGGS as the fact is it's a delicate book and you don't want anything spilled on it or for it to be soiled in any way (it's valuable in spiritual terms and monetary terms).
That is interesting..in a way..imho its lost in the sands of time just as asking when did the first shirt..or first pants..began to be worn ?? When did the first human began to cover his body...???
The Gurus used to be seated on a high platform in snagat..with a "vishhaai"..that is bedspread/and also chaur and chandnni....etc especially the Gurus Hargobind Sahib and Guur Gobind Singh Ji whose Darbar rivalled the Court of the Mughal Emperors...the Battle fo Bhangganni (Guru Gobind Singh Jis first battle) came about becasue the raja of the Hill state nearby to Ananadpur Sahib, envied a Beautiful Parsadee hathi..a Trained Elephant that could BOW, SALUTE, Wave the Chaur sahib etc etc and was presented to GGS by the Raja of ASSAM. The Hill raja was extremely JEALOUS that he was the raja and that a mere "guur" had this gifted to him...so he declared war on GGS to rob the elephnat as wella s afew more extremely Royal Gifts.
2. When POTHI SAHIB was parkashed in Darbar Sahib by Guru Arjun Sahib, it being accorded the exact same status as the Living GURU ( From that day onwards Guru Arjun ji refused to sleep on a BED and began to sleep on the floor beside Pothi Sahib )..and since all the other things like chandnni (chandoa), chaur palki etc were already in place, the vichhaii (chadran bedspreads etc used to wrap Pothi Sahib..and the RUMALLAH must have been invneted to cover the Parkashed Pothi sahib..ita only Natural as the uncovered open book of a size of a GRANTH would be unseemly and "dangerous" as well..inviting damage of sorts, tearing of the pages etc etc.
However all this is such a NATURAL thing to do..no one has mentioned it as its COMMON SENSE !!..as when did Guru nanak ji tie his frst turban ??