I think perhaps it's not a good idea to throw away anything. Rather, a company should start from scratch while still utilizing their existing processes. Do a complete overhaul in the background, and when your plan is complete then implement it.
Now, why would a company do this? I suspect it might be helpful to do if there has been no reevaluation done for a set number of years. It may also help to bring in fresh eyes from outside the company to make sure that everything being done adheres to current best practices. I stress the word "current" because some companies let things sit for too long, and fall behind over time.