The free plan tells you what the way you already store a pair is costing it. It also gives you, in full, the two parts of a reading that could send you off doing something unnecessary if they were hidden: the pairs that need no attention, and what the reading could not know.
Those two are never behind the paywall and are never obscured by the free-plan watermark. Covering them would be the one genuinely harmful thing this product could do.
What you pay for is the comparison — all four methods at once, the step diagram for the easier one, and a print sheet. Knowing that something is slightly bad changes nothing on its own. What changes behaviour is knowing what to do instead and knowing it takes no more doing than what you do now.
The moment it pays for itself is the changeover, when a drawer goes away for a season at once. That is the case where duration is long enough for the answer to differ, and it is the case where doing it a better way costs you nothing extra.