To minimize human error TB Yoma 56

 The High priest has to sprinkle the blood of the bull one time upward and seven times downward and then he has to sprinkle the blood of the goat one time upward and seven times downward. The blood of the bull was in one bowl and the blood of the goat was in another bowl. Obviously, the High priest could not hold both bowls and sprinkle the blood at the same time. He had to put one bowl down. Because the bowl came to a point, he had to place it in a pedestal so the blood would not spill out. In the Mishna Rabbi Meir and Rabbi Yehuda disagree on how many pedestals there were. “The High priest then emerged from the Holy of Holies and placed the bowl with the remaining blood on the second golden pedestal in the Sanctuary. Rabbi Yehuda says: There was only one pedestal there, and he took the blood of the bull from the pedestal and placed the blood of the goat in its place.” (TB Yoma 53b, Sefaria.org translation)

Why does Rabbi Yehuda require only one pedestal even when “he is of the opinion that one may rely on writing, as proven from the halakha of the collection horns (tzedakah boxes in the Temple-gg) If so, on Yom Kippur as well, let us place two pedestals and write on them which one is for the blood of the bull and which is for the blood of the goat.” (Sefaria.org translation) The answer is simple. Rabbi Yehuda wants to minimize human error on Yom Kippur.

“The Gemara answers: The reason they did not place two pedestals with writing on them is due to the High priest’s weakness. Since he is fasting during the entire day’s service, the writing will not be on his mind; he will pay no attention to it and might become confused. As, if you do not say so, that there is concern for the High priest’s weakness, even without writing he should also not err, as this bowl in which he collects the bull’s blood is relatively large and this one for the goat’s blood is small.

“And if you say that he does not collect all the bull’s blood but only some of it, so that the bowls are of equal size, didn’t Rav Yehuda say: One who slaughters the bull must receive all of the blood of the bull, as it is stated: “And all the blood of the bull he shall pour out on the base of the altar” (Leviticus 4:7).

“And if you say that perhaps some of the bull’s blood might spill, yielding equal amounts of blood, there should still be no mistake, as this blood, that of the goat, is white and bright compared to the blood of the bull, and this blood of the bull is red and darker than the other. Rather, the reason must be that due to the High priest’s weakness, these differences will not be on his mind. Here, too, the writing will not help, as due to the High priest’s weakness the inscriptions will not be on his mind.” (Sefaria.org translation)

Remember, the High priest stayed up all night erev Yom Kippur to review the correct procedure. On Yom Kippur day only he could perform all the rites and that was a lot a work. As the day went on he must have gotten more and more tired. When you’re tired is when mistakes creep in. To avoid making a mistake of picking up the wrong bowl of blood, Rabbi Yehuda insisted that the High priest always hold one bowl.