Sometimes, we can't measure what we need, so we invent a proxy, something that's much easier to measure and stands in as an approximation.
TV advertisers, for example, could never tell which viewers would be impacted by an ad, so instead, they measured how many people saw it. Or a model might not be able to measure beauty, but a bathroom scale was a handy stand in.
A business person might choose cash in the bank as a measure of his success at his craft, and a book publisher, unable to easily figure out if the right people are engaging with a Citeşte mai departe...