So that people can still enjoy the benefits of Premium license (and not trend to downgrading to the Free license) after the GAOTD 1-year Premium subscription ends.
I feel that after 2-3 years of being Premium users (e.g. keeping Premium license via fulfilling TrialPay-style offers, etc), users might just decide to buy lifetime licenses. My idea also helps Your company get more revenue from users (via TrialPay style offers) - users who would otherwise have simply stayed on as Free version users and not upgraded to Premium annual or Premium lifetime.