Probably the most important reason for the rule is, it prevents arguments and long, drawn out discussions that eventually lead right back to each persons original opinion.
You may consider a stated value as perfect, while I consider it a rip off, or great bargain. We could beat the subject to death, with lots of examples and justification. Neither of us would be wrong, so neither would change our mind. But, we would have wasted a lot of time and bandwidth.