The 3rd event is for emissions and power both. Multiple injection events are a fact of life with modern diesels, it is the way they generate the broad power bands while meeting emissions. The early 6.7's used up to 5 events and I think the later ones could have 7. Engines like the VM with Piezo can utilize up to 9 events.
More events will almost always increase wear and decrease MTBF, but, construction of the injector has progressed also so it is not as bad as you think. Fuel quality is a much bigger contributor to failure than number of events.
With the 5.9 electronics the 3rd event is more of an extension or post pilot event. EGR is intended to reduce cylinder pressures and temp that increase the generation of NOX. Given the fueling curves that is only an issue in high load higher rpm situations. The ECU is not monitoring temps so it uses engine load and rpm to decide whether that event is needed. It also extends the power band out to 2900 rpms for higher rpm operation when load does not warrant it. As rpm increases fueling increases as does timing which sets up conditions for NOX creation. You add a little more fuel to the end of one event to maintian power level and the late burn also robs some oxygen from the next event to balance temps and pressures.
That is with stock fueling, modify the fueling curves and it all goes out the window. The Smarty reportedly has turned off this event many years ago but it is extending the duration adn modifying the presurre curves so it is not real important in that repsect. Doing that is looking for efficiency not emissions so it is a different paradigm being used.
Isn't the third event the dreaded "cylinder wash" that they claim puts fuel in the oil ?
No, that event is only activating when cylinder pressures and temps are high. There is no chance, given everything is operating correctly, that that will cause fuel in the oil. It does cause more soot loading but not fuel.
What the 3rd event, really any emissions tune, does impact is efficiency and soot loading in the oil. The idea that it doesn't "hurt" anything is ambiguous at best. It doesn't hurt the engine operation, but, it does hurt efficiency and ultimately fuel economy, it does hurt the oil in the form of more soot that leads to accelerated engine wear. However, oil compostion and engine material composition has been adjusted to compensate so the lines of what is good and bad and their effect is blurred.