It's most likely there so you've got situations like:
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Get tempted to buy something from cash shop, can't afford subscription with the gralats I have left, upgrade gralats to pay for subscription. Could afford both straight off if it were my own currency involved, but this way I'm spending more money.
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Quote:
Oh man I have a few spare gralats left but just not enough for this item, guess I'll buy (way more than I need) so I can get it. Rinse and repeat.
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It's done in most other games too, even in places like the Xbox Live Marketplace and WiiWare store.