I want to use my imagination and cleverness in setting up the plot, the challenge and the development of the characters and setting. I do NOT want to use my creativism and imagination in trying to determine whether the fireball can reach around the corner to hit the gnoll or whether using a double move to get past an ogre with a club opens yourself up for attack or not.
In short, for the things specified by the rules, the way "normal things work" (where normal can include things like the most common spells and magic items) I do not want my imagination to be needed. I want both my players and myself to draw on the same understanding for normal things, how they work and what the expectations are.
This way, when the fireball FAILS to stretch around the corner and toast the gnoll and this goes contrary to my players and my common understanding of the "way normal things work", then my players can IN CHARACTER go "hey, wait a minute, What's up with dat?!?" and recognize the event as a CLUE that something more than "normal" is going on.
this does not happen as readily, I cannot be as subtle, if the basis for resolving the question of "does the fireball wrap around the corner" is "gm judgement call spur of the moment how does he feel today"...
Knowing how things "normally work" because they are defined as such in the rules, clearly so in the best cases, does not in any way mean that i am somehow hamstrung or prevented from creating, using my imagination, UNUSUAL situations where things are not "normal" and thus having special and wondrous events unfold.
In short, having rules that tell me what is "normal" does not limit me to running only "normal" events, scenes and stories. It just helps both my players and myself recognize the unusual ones and appreciate them.
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As an aside, a slightly different but related subject...
If the "normal" defined by the rules in a given case is unacceptable to you for being "normal" for your campaign, then you should make a campaign specific setting change ahead of time. For example, DND3e has raise dead et all as "normal" spells for clerics to get. This makes death much less serious a thing for the campaign world and affects the issue of mortality greatly. Another example is GMW. This spells makes enchanted weapons a "normal" thing as several classes gain at failry low levels the ability to provide enchantments to weapons common and lasting for long periods. Another example, fly spell is fairly low level and provides rapid flight over a decent time frame.
These are all "normal' in DND3e, part of common spells lists.
If you want to run a world or campaign or story where death is critical, where magic weapons are rare and wondrous items, or where flying carpets or winged boots are exotic and wondrous things... then you need to address the "normal" availability of these spells. A +2 sword cannot be a rare and mystical beast that character go all gushy over if the sorcerer has been boosting blades for three months now. A pair of winged sandals wont be seen as a treasure above others if the sorcerer is hovering right next to the winged boot guy looking bored.
So, make sure the "mormal" is what you want as "normal" and then spend your time and energy and creativity on the interesting stuff.