When should I look at the benchmark vs. when at the predicted sales ?

Short answer: 

If you want to explore the growth opportunities for your current network, you should analyze the benchmark, which can be found in the Location Performance module.

If you want to know what's the impact of openings, closures or relocations on your current network, you should calculcate & analyze the predicted sales by using the predictive model in Location Planning. 

Why are the predicted sales of an existing location different from the benchmark of an existing location? Because the predicted sales take into account a reality correction factor, to reflect reality. This is why the predicted sales of existing locations that are over- or underpeforming are never equal to the benchmark!

Read also: how to use reality correction factors.