Mental note of what training "worked" and what did not I guess.
It is not that simple though is it as you might do a mega session one day and see an immediate "dip" but then get a super compensation effect few days later.
Also it is impossible to keep your training "Productive" indefinitely but equally sticking on "Unproductive" for ages should ring alarm bells.
Personally I view it as a useful check that things are going vaguely in the right direction.