The curtain is what gives the Aron Kodesh its respectful appearance. In the Mishkan (Tabernacle), the curtain separated the Holy of Holies from the rest of the space, which was on a lower level of sanctity.
It is exactly this division that allows us to properly receive the light of holiness. The curtain is always over the Aron Kodesh and conceals the Torah scrolls in reverence for their holiness.
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