> Why would you purchase a $500 gift card for yourself to "keep a subscription without worrying about it" as opposed to just paying the small monthly amount? Honest question, I literally don't understand that motivation at all. In my mind a gift card is more problematic than a normal credit card in this scenario since it eventually runs out.

Asides from the promotional bonuses that other users have mentioned, if you have an Apple Family Sharing group you can only use a single credit card tied to the main account for any payments to Apple, but individual accounts will draw down from their Apple Account balance before using that credit card - so gift cards let individuals pay for their own Apple things (subscriptions or otherwise).