Waheguru Ji Ka Khalsa Waheguru Ji Ki Fateh
Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/sikh-gurus/23126-after-rehraas-sahib-harmandir-sahib-amritsar.html

As far as I was aware, the reason is a purely practical and scientific one.

Harmandir Sahib is surrounded by water, making the air very humid. The effect of humidity fluctuation in the air has a deteriating effect on paper. The five rumallas protect Sri Guru Granth Sahib Ji angs from this effect.

So in this particular location, it is not due to ritualism, but due to practicality.

When this is copied elsewhere without the need or knowledge of why it is done then it becomes ritual.
Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/showthread.php?t=23126

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