Why is the gap concept exclusive to tournamnents?

[Koss]

So I'm pretty sure I understand the idea of the gap concept, and most posters here seem to regard it as more of a guideline than an actual rule, but what makes it apply in tournaments and not side games? It seems like in ring game you still don't want to be calling with hands that are weaker than your opponents average raising hands. This also goes against the idea of calling raises with good implied odds hands, which can clearly be a +EV play if done right, even in tournaments. I guess I just don't see the big idea of the gap concept. It seems like a pretty basic idea that can be deviated from if the conditions are right, regardless if it's a tournament or side game. What exactly am I missing here?