Why post to specifc sessions?

I have a POE based app that I find I am adding new sessions as I add
functionality.  I am at the point where a new session generates an event
of interest.  So, to let my ui get the event (from the new session):

	$kernel->post( 'ui', 'change_series', 'Auto' );

Since in my 'ui' session I need to define the handler, why do I need to
indicate the target session in the call to post?  Why can't I just send
it into the event loop and have any session configured to handle it do so?

Thanks,

John