Only reason to buy one, ever, is in certain situations for credit card churning when you need to meet a minimum spend, or if you get bonus points through some other rewards program. I remember a few years ago, there were people making bank by buying up Visa gift cards to earn sign-on bonuses with credit cards and earning back the gift card fees on gas miles. Most grocery stores no longer give the gas rewards on gift cards, though.
In any case, I've bought them a couple times to hit a minimum spend before a deadline, but that's it. Never had trouble spending them, but they're a pain to keep up with.