When I buy a pack of tokens rather than depositing USD or similar into an account, I'm more likely to spend all of those tokens. I'm also not likely to say "hey, I demand you give me my remaining balance back" after buying a single cheap item. It also makes giving rewards easy and makes sure that money will be spent in the same place.
Some free to play games, World of Tanks for example have larger things that are sold both in gold and in USD - because they make it possible for people to earn gold in game through multiplayer competitions but want it to be easy for someone who just wants premium time to buy it with cash.
In my experience, it's exactly the opposite, since packs of points/tokens are not typically divisible by the price of common purchases. For example, you might have the option to buy 500, 1500, or 3000 points; items then cost something like 740 or 800 or 1600 points. To spend your last point/token you almost certainly have to buy more than you originally intended.
They're perhaps not the reason you bought the premium currency, but they're worth enough that you'll spend them.
Me too, but I guarantee we're outliers. You probably look at your credit card statements sometimes too :)
They could easily choose to make the exchange rate easy (at least in e.g. USD), or even better, forego the fake currency system entirely and allow transactions in the user's local currency. Instead they erect multiple layers of obfuscation to keep you from thinking about the idea that you're actually spending real money. Something tells me they spend the resources on this system because it results in increased net profits.