Not sure how buying a gift card will be any great savings vs. using a credit card, unless as you say, you could easily convert it to cash.
All it does is allow you to front-load some spending that would otherwise take a few weeks/months. If you have a program-affiliate AmEx, it's a great way to hit spending thresholds. Last year AmEx had a promotion where not only could you buy the cards fee-free, but if you bought five or more, they would waive the shipping costs. I bought five $500 cards to hit the $25,000 spending threshold on my Skymiles AmEx, which gave me 10,000 MQMs and GM status. Well worth it for me last year.