Supposedly, adding supplies to the chests improves the armor of your forces when they support you in the final battle...maybe. In practice, they don't seem to make much difference. However, by that point in the game, you might as well donate what you can--you won't be using your gold for much else.
Also, donating supplies gets you a fair amount of experience. Donating a stack of 99 elfroot, for example, nabs about 800-900 xp. And there's a vendor in the Dalish Camp who sells an infinite supply of elfroot for very little gold.
Whoops.
My suggestion would be to throw in all your extraneous supplies through the course of the game (to keep your inventory from getting cluttered with junk), and then a bunch of your extra gold just before heading out to the final battle.
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Now Playing: Two Worlds II