Typical demand: Set a high price, and less people will buy your product. Set a low price, and more people will buy your product. Your income is the number of buyers multiplied by the price paid. For any market, there is typically a sweet spot somewhere in the middle, that is, a price you can set that maximizes your income.

Now, this sweet spot may vary from market to market. If you are forced to set a single same price for ALL markets, it is likely to be less-than-optimal for most of them. So you're losing potential income in most markets. So it is to your advantage to offer different prices to different markets.

Taken to extremes, each individual person in the world is a market. You make the most money when each buyer pays the maximum price they'd be willing to pay! Why? Because you can maximize your number of customers without reducing the high price that a select few of your customers is willing to pay. Ted will pay $50, Fred will pay $30. A global price of $50 earns you $50 (Only Ted). A global price of $30 earns you $60 (Ted and Fred each pay $30). If you can instead divide the market and price for Ted and Fred independently, you'll make $80. Of course, to be this selective about pricing has costs. Someone needs to evaluate each customer and what they'd be willing to pay. For something like bread, it's not worth it. But for more expensive items... a used car salesman-- it's his job to evaluate your maximum price as best he can.

Anyway, it's generally to your advantage if you can offer different markets different prices.

edit:
Muddled some numbers.