These adjustments make the cab customizable to individual tastes and goals for how they want a preset to sound. Not unlike Bass, Mid, Treb EQ adjustments on an amp. "Well, if the amp is a great amp, why would you need to use an EQ on it?" I'm not sure anyone would ask this question, but the High/Low cuts on the IR work the same way.
In fact, I often find myself tweaking the High/Low cuts more than the amp's EQ.
I agree with comments above too. If you're playing through a PA or FRFR, the nasty highs and resonant lows simply don't sound like a real amp to me. Traditional cabs have a very narrow frequency response compared to FRFRs. There are no rules, but if you want your preset to sound as traditional as possible, I don't know how you can get away from the High/Low cuts.