It does allow you more flexibility with pricing. As an example, iStock sell photos that cost 1 credit or 3 credits depending on how fancy they are. But the credits cost a variable amount depending on whether you bought a few credits or hundreds. So they can offer a tiered a la carte pricing structure that also has bulk discounts. Hard to tick all those boxes without credits.
Multiple companies I've used simply offered me a bonus in $/€ when I made a larger deposit, say, paying 20€ gets you 20€ in your account, but 40€ gets you 44€ (bonus of 10%).
