As the title says, you have to charge it to your room to use it, but if they won't let you charge to your room, you can't use it at all. Yesterday I was at the IHG on Page and wanted to buy some water and snacks from the minibar, but they only took card and cash. I feel like IHG's biggest advantage is all those big roadside motels in the Midwest, but a lot of those places aren't very by-the-book, so if you can't use it at many of them, even the $25 FN might not get used.
At high-end places like IC, $25 isn't even enough for one person's breakfast, unless you're back in China, then it's fine...
Some hotels may specify that the F&B credit can't be used for the minibar, so it's best to ask before you use it.