A few reasons the one already pointed out by ElDevilDog being a first choice.
1. Less risk of personal info theft
2. Financial privacy in a domestic environment
3. Availability (grocer's, gas stations, box stores, online refill...)
4 . Convenience (available 3 packs of $10 cards or single $15 and $25 dollar cards)
5. A way to buy in game items for those without bank account or CC access
7. A way of budgeting spending
8. As the monicker implies to give as gifts to friends in the game
Subway giftcards aren't the only method so for me I would say number's 3 and 4 are the dominant reasons.