The Romans introduced set forms of action, following the example of the Greeks, and established a rule that each cause should be conducted by its proper action. This was necessary in their manner of judging; it was necessary to fix the state of the question so that the people might have it always before their eyes. Otherwise in a long process the state of the question would continually change, and be no longer distinguishable.
—Montesquieu. The Spirit of the Laws, 1748.
Defendants must be able to prepare their defense. To do this they need to know what the case against them is. This requires that the plaintiff or prosecutor give a detailed written account of the facts they allege are essential to their claim, and the legal ramifications of those facts. It is the mark of tyranny, in both civil and criminal proceedings, when the case against the defendant is not fixed.