Yelling just lets the kids know that they are in charge.
yelling gives them that extra couple seconds to think about what they did while they wait for the spanking to come.
Actually, a raised voice - used judiciously - conveys the urgency of a situation. As an example, if my own kids have been trained to obey commands uttered in a normal voice, then raising my voice lets them know that they are to obey immediately, without question and without delay.
Unfortunately, yelling tends to become a substitute for "making" the child obey. For a younger child, "making" them obey often means taking them by the hand and guiding them physically through the commanded actions. For an older child, "making" them obey often means ensuring that they do not enjoy life until they have complied - whether through punitive chores, removal of priveleges (my son hates it when I take away his computer priveleges), or similar.
But enough of my rambling...