One benefit is that they only need to adjust the price of points in other currencies as they fluctuate, and not the price of every single item they sell.
At one point when the Canadian dollar was nearly equal in value to the American dollar, Sony adjusted all their Canadian prices on PSN to the American prices. That was only possible because they happened to have prices for all items in another currency of approximately the same value. Now that the Canadian dollar is weaker again, they will be making less from Canadian sales.
It's a choice between trusting the value of a new currency, or coming up with prices for all items, in all currencies, and adjusting them over time.