Why use Allselected with Addcolumns?

Hello, team. This is not regarding a specific scenario, but I have been seeing Allselected being used as :

e.g.        Addcolumns( Allselected(Product'[Brand]'),  "Amt", [Avg Sales]) )

I don't understand why we would necessarily use allselected at all in Addcolumns. What if I used All(Product'[Brand]') ?

Thanks.