who know's, there might be an occasion where someone looses access to their accounts and their only method to regain access and secure them is through a Protonmail set-up, they likely would have set up a paid account, however under severe cyber attacks they may not be able to access there usual avenues to pay their arranged debts, therefore a deferit option provides not only the option to show goodwill that you are going to make the payment, but also a safety net and sometime to regain access and secure what needs to be.

Thank you