The rules must be specific enough that it takes less than a few seconds to decide whether a post breaks them. When you report posts like these, you must quote the local rule that was broken. Rules will be ignored if they are not specific enough, or if it takes too much work to enforce them.
It is possible for someone to create a "ruleset" that others can quickly apply to their topics by clearly linking to the ruleset.
(It has always been my policy to enforce local rules, though not many people have created topics with such rules.)
For a more recent addition and clarification on the intended use here is one from a locked topic
Overall it's not meant to hinder any discussion. I see it as a way to create a certain environment for your topic, whether it be a focused discussion, a giveaway/contest or a general fun thread. Where there is a central point or theme that needs to be followed or used as a guide to participate in the intended way.