I would suggest that the existence of official lore, done right, serves as a sanity check for mechanics and rules because it requires the consideration of
consequences and encourages a bit of thought about integrating new additions. One could certainly take a different tack from what would be official, but it'd good if such thinking actually at least
happens; and if GMs could read such and consider in advance how
they would treat such aspects, rather than potentially be taken by surprise if a curious player asks a relevant question during play.
For D&D 5E, for instance, building a coherent setting would necessarily need to consider the prevalence and use of magic. If magical abilities aren't that unusual, for instance, then there's a lot of spells and magical items whose existence would cause significant divergence from a clichéd
mundane quasi-medieval-Europe setting. Even relatively low-level magic like
mold earth, mending,
spare the dying, alarm,
comprehend languages,
cure wounds and so forth; never mind still somewhat-modest spells like
detect thoughts, clairvoyance,
sending,
plant growth and
zone of truth. Everything from law enforcement to (counter)espionage and security measures to agriculture (and perhaps population density!) to construction work would be seriously affected. If nobles and authorities have to worry about the occasional intruder protected by
invisibility, what security measures might be common? If adventurers are attempting to infiltrate the mansion of somebody they suspect of some secret evil, and they find an portal secured by
arcane lock, or if they stumble on a
glyph of warding -- is that by itself unusual and perhaps suspicious?
Are those without magic frequently protesting to restrict or expel magicians for fear of being rendered obsolete to the degree of not being employable?
If magic is
rare, then how do authorities handle dealing with it given what it can accomplish? Are the few with talents "strongly encouraged" into serving lords who both crave and fear those abilities? Are diviners hunted down, for fear that they'll spy on the Powers That Be; never mind the fear and resentment that enchanters would likely evoke, were it known that they could
command or
dominate person to suborn critical personnel? Is a
wand of fireball a highly illegal bit of contraband, or maybe something that will be requisitioned for military purposes if authorities find out about it?
Is the existence of
subtle spell known, and if so, might authorities view it as dangerous knowledge worth suppressing or otherwise regulating due to how it'd make it far easier for a malevolent caster to exploit his abilities without detection?
...
For that matter, if a single night's rest heals the vast majority of non-fatal injuries, that too might have serious consequences in everything from warfare to workplace safety to the development of healthcare services.
