What it determines
Your break-even ROAS, your maximum allowable CPA, whether a campaign is winning or losing, and whether paid traffic can work for the product at all.
Why agencies skip it
Because asking it invites the answer "the maths does not work", which is a harder conversation than promising a 4x.
What we do with it
Set the break-even in the account so every report judges against it, and tell you honestly if the product cannot carry a click. The calculator does the arithmetic if you want it before talking to anyone.