The JCL SPI can be configured to use different logging toolkits (see above). JCL provides only a bridge for writing log messages. It does not (and will not) support any sort of configuration API for the underlying logging system. Configuration of the behavior of the JCL ultimately depends upon the logging toolkit being used. Please consult the documentation for the chosen logging system.
In short Apache Logging is a wrapper for other implementations of logging, and one being log4j. So you still would have to configure log4j, however your code could switch to another logging package in the future without you having to change code. In particular it is probably good for frameworks systems where the framework writer can't be sure of the underlying logging api used by framework users.
In other cases it is probably sufficient to have your own Logger class that fronts log4j and so gaining the flexibility of moving away from log4j should you choose, and yet not adding the apache logging dependency (and learning curve) to your code.
Using this framework the underlying logger implementation can be changed.Its finds its usage in third party libraries which plugs and becomes part of some application and in that case it can use the logging framework of the application as this can be configured.