The reason that kettle pH is high, is that it helps in the conversion of SMM to DMS and it helps with the utilization of hop bittering components. When you are brewing a beer with little or no pils malt (it has high SMM content), there isn't really a reason to keep the kettle wort pH high (say around 5.4). When brewing without pils content, then its OK to target a kettle pH lower than 5.4. The only drawback is the slight reduction in hop utilization.
Why drop pH? Well you do want your wort and subsequent beer pH to be below the upper 4's as soon as possible to assist in knocking out beer spoilers in the wort. Although yeast are the ultimate arbiters of where the beer pH will fall, helping out in getting the pH down may help the beer achieve a lesser pH. I'm not sure that's always a good thing, but its a thing.