It's not even that. It's the interface that other logging/tracing apis can plug into. It's the "SLF4J" of tracing/metrics gathering.
Why is this useful? Because it lets people writing libraries provide the ability for users of their libraries to track performance information without dictating to them what performance tracking tools they should use.
Why is this useful? Because it lets people writing libraries provide the ability for users of their libraries to track performance information without dictating to them what performance tracking tools they should use.