Logs are an ultimate source of information to the health of an application. Logging data helps understand if something has gone wrong and what might have gone wrong. The problem is that there are two sides to logging.
Developers provide the first level which are the log statements themselves. I have always found that a major/minor log code (i.e. (error/warn/info/trace = major; minor = module/method/logic) to be the most straight forward way to provide log information. Identify the type of log message being recorded to a specific category.
For the operations side of the house they can monitor logs and based off of known major log codes they can decide if they need to take action. A major code that indicates an error can be immediately dispatched to a field technician. While other levels can be monitored or collected if in troubleshooting mode.
However, if an application development team does not provide any well defined codes for log messages and only provide free form text then the operations/runtime side has difficulty determining if a specific log entry is a problem or not. Unfortunately, most application development teams choose the free form text logging as their solution which makes runtime operations that much more difficult and, in many cases, confusing.