No what I want to understand is why is there an option to charge additional cost for the zip file? Would it not make sense to just charge the entire game and get the files?
That is why. By default, yes, you set a price for the entire project, and buyers have access to all the files in perpetuity. But sometimes creators want to sell tiers: additional items with more value. This is a simple way to have them all be part of the same project instead of having to create lots of new pages and struggle to make it clear they're related.