To clarify: let's say you were planning a game. Would you carefully select, from one of the hundreds of game systems available nowadays (from the popular to the barely known), whatever system is a "best fit" to the content of the game you have planned, or would you instead take your favourite system - or even just whatever you have lying around - and modify the system to suit your story? (Leaving out for the moment the option of modifying your story to suit the system.)
I ask this because I feel I've noticed that people are very quick to suggest that someone look at a different game when they want to change rules to better suit a given story, particularly in online communities. Why is this?
Edited for grammar and clarity.