A visitor at our Yom Kippur services last year asked why we take all the Torah scrolls out of the Ark and have members hold them during the chanting of Kol Nidre. Some of the scrolls are large and heavy - to the point that we avoid using them for the weekly reading - and Kol Nidre is chanted three times.
One reason, and I think that this is probably what occurs to the most people, may be to emphasize the seriousness of the occasion. Our congregation also has a Torah scroll (just one or two, depending on the Torah readings) held - usually - by a member during the prayers for the country, the State of Israel, and so forth.
But I think that the custom originated centuries ago as a result of an iniquitous practice in Europe called the Jew's Oath.
In the Middle Ages, church and state were one throughout Europe, and oaths taken in court typically required swearing on a (Christian) Bible--something that persisted almost to the present day and may still occur in some jurisdictions. A Jew would not swear on a New Testament, or repeat an oath invoking the Trinity.
So an alternate oath was demanded. This was established in the Byzantine empire by the tenth century, although it did not immediately become universal in western Europe. There are numerous formulations for it from the Middle Ages and later, commonly involving holding a sefer Torah, wearing a crown or girdle of thorns, standing on the skin of a pig, and other indignities, some verging on torture. This version from Frankfurt in the fourteenth century is an example: "The Jew shall stand on a sow's skin and the five books of Master
Moses shall lie before him, and his right hand up to the wrist shall lie
on the book and he shall repeat after him who administers the oath of
The Jew would then call down on himself some or all of the curses in the Torah. In some places, such as Arles in France, a thorn branch would be pulled "between his loins" while he did so.
Another reason for the more Judaico was that authorities distrusted the word of Jews because of the Kol Nidre recitation, which (read literally) annuls all vows that might be made during the coming year. From a Jewish point of view, the reason for this was to prepare for vows that might be made under duress, probably including false conversions; we also understand it as applying to vows that a person should never make, such as when an exasperated parent says to a child, "If you do that again, I swear I'll kill you." We might also interpret as covering vows that we made in good faith but just could not keep.
The correct reading of the Kol Nidre text should have been a reason for taking the word of Jews seriously: it shows that our ancestors considered every oath valid, even if made under duress or extreme mental strain, unless it was relieved by Kol Nidre.
The Jew's oath started to fade in the nineteenth century. In France, a rabbi was prosecuted when he refused to open the synagogue for it--and he was acquitted. Some German states (before Germany was unified) dropped it in the 1820s and 1830s; Zecharias Frankel published a commentary when Saxony discontinued it in 1839. Prussia did not completely abolish it until 1869, and it persisted longer in eastern Europe, having been demanded in Romania as late as 1902.