That's what I did. I figured it would be really convenient then.
Yeah I'm not much of a coffee drinker but figured I could get snacks there. I wish I would have thought of it last time we went. I get the gift cards free with Bing, so makes for a cheaper trip.
You can also put all of your gift card money (including eGift Cards) onto one gift card as well, if you didn't use the app that
gmi3804 suggested. For example, if you had 2 or more gift cards you could transfer the balance over to one card so that you're not carrying and trying to keep up with multiple cards.
With your gift card you can get snacks, seasonal snacks/drinks, sandwiches, breakfast sandwiches, coffee mugs and any other edible or non-edible items that Starbucks normally sells at DLR. If I didn't buy anything edible at Starbucks I would get a mug or a magnet or something like that instead.
I
am a big coffee drinker, though I am
not a big Starbucks customer at all!

So, over the last year or two, as I have begun to accumulate Starbucks gift cards from various sources, if I have not used the gift cards at my local Starbucks then I have set the gift card aside to use at DLR when I'm there. The last time I used my Starbucks gift card at
Disneyland Resort I think I got a slice of lemon loaf and a cookie from the DTD location, and a slice of gingerbread loaf and a peppermint drink from the Market House!
Even though I am not a big Starbucks customer, every so often they do have good deals/treats for their Starbucks Rewards customers that I will take advantage of (free birthday items, or a 2-breakfast-sandwiches-for-the-price-of-one deal [so I got 2 decent sandwiches for a total of $2.95], for example), but I
don't think those Rewards deals/offers can be used at any of the Disneyland Resort locations.
However, I
think that using a registered gift card at the Disney Starbucks locations will contribute to accumulated points and stars on your Starbucks Rewards account, which can be used for future deals and offers outside of DLR.