So let's say you make some hardware changes. You add a ported head, for example, or maybe install a hotter cam, a larger throttle body, or something else. Now there is more air flowing into the engine. The computer does not know this but can still compensate in normal driving from the O2 sensor's data. Once they get high enough, things start to melt.
Pistons are typically the first thing to go.
Avoiding that situation is easy. Just install these two gauges and pay attention to them. Use this info and make your own call. Whatever you decide, be consistent and comfortable with it. For that reason, install these gauges while your car is still stock .