When do we use logging? This very technically-oriented question should be rephrased this way: when do we provide information about what the application is doing? This is exactly what logging is used for; it lets the user know what the application did at some point.
This is something we, as developers, often forget because we are focused on the feature we are implementing. But if the application intends to hit production, it is crucial to not forget to make sure the operation teams can work with it very efficiently as well. Don't forget that if the development takes six months, the production will probably last for some years and the cost of an incident is way higher than a small delay before the production is launched. Therefore, investing in a system communicating enough information is often worth it.
However, logging is not a trivial task to underestimate...