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 book, might rely instead on a rank on a single bestseller list. One last example: the non-profit that uses money raised as a proxy for difference made.
You've already guessed the problem. Once you find the simple proxy and decide to make it go up, there are lots of available tactics that have nothing at all to do with improving the very thing you set out to achieve in the first place. When we fall in love with a proxy, we spend our time improving the proxy instead of focusing on our original (more important) goal instead.
Gaming the system is never the goal. The goal is the goal.