If you measure and optimize towards some meaningful metrics, you will make decisions with this metric in mind. This could have unintentional side effects.
An example is when you try to optimize for conversion rate, where you try to monetize every visitor, you might end up lowering the ‘customer loyalty’. This could be because, you have new customers that aren’t actually convinced or got fooled, or that the service you’re providing is hurt by these conversion optimizations.
One should really be data informed, not data driven when optimizing, as metrics can be harmful.