If I have an accounting system I like and can navigate, what is the benefit of the Profit First method?

The Profit First system is designed to work WITH accounting systems such as QuickBooks. It is not a duplication but rather an enhancement because QuickBooks just shows you what comes in/out of the business and when. The Profit First cashflow management system gives businesses visibility to how money is saved and spent and provides additional clarity that cannot be seen on financial statements.