It's one of the most important things.
Making things more realistic ruins games. Changing things to have them "make sense" destroys fun.
I've written and designed computer games before and the most important lesson I learned from those experiences was to design fun mechanics and make the game about that fun. Jeff Vogel talks about it here.
Every time someone suggested a way to make the game 'more realistic', it never failed detract from the game. Add armor damage and wear and tear on weapons causes tedium. Make the monsters fight each other causes endless messages and rooms full of dead creatures. How about at a table? Making people remember to eat, go to the bathroom and feed horses? You've insured that the players recite a list of items at various intervals. Sounds super fun, right?
Why is it when people sit down to play or run role playing games they forget that they are playing a game? Is Jenga less fun because you don't use a crane? Is Monopoly less fun because you can't buy stocks or put your money in a bank? Is Risk less fun because you don't use supply lines?
Games have rules and mechanics. These are supposed to generate fun on their own merits. Over and over again I see people change games for the sake of 'realism' or to 'make sense', resulting in something that isn't fun just for its own sake.
Take a game like Old School Hack. Not only is a beautiful piece of work, it's also super fun to play. It is not a traditional play the same character forever type of RPG, and the turn structure is certainly different then normal. When a friend of mine ran it, she wanted to change it because it wasn't what she was used to, because the new system didn't make sense to her. Trying it out we found out it was fun on its own merits.
Ask yourself, devoid of setting and flavor, is what I'm doing an enjoyable activity? This is not a subjective question. In a game, an enjoyable activity comes from making choices with significant consequences. I've been talking at length lately why combat is an enjoyable activity and rolling for skills is not. If you blocked out the pictures, flavor text, and names on the cards in Dominion, the game would still be mind-blowingly good.
Realism in game design is an easy trap to fall into, but it invariably results in poor, boring games. Ask yourself why you are making any change, and if the answer isn't "this is objectively more fun because it provides either more choices or choices with more significant consequences resulting in more interesting play for the players," then don't make the change.
How will you ever know if you like something new if you just change it into what came before?
