@dbu See the methods defined in the interface (and approximately all my comments in the different issues about logging in PHPCR).

It is a logging interface tailored to the PHPCR. domain This allows to do more than just a basic logging (which is what is done byt the PSR3Logger implementation just after).
The same logging hook can be used to collect calls for the Symfony profiler (see the DebugStack implementation above) for instance