This article on sub-10 cent MCUs mentions that many of them use a high voltage programming interface. What is this, and why would a product use it over a "regular" 5V or 3.3V programming interface?
EEPROM technology uses a high voltage to inject charge into the ‘floating gate’ cells in the device. The floating gate charges are what define the 1’s and 0’s that are read out later at normal voltages.
If the chip doesn’t have its own high-voltage converter on-chip, it will require an external elevated voltage to be supplied to it during the programming sequence.
Why do they make such a thing? The on-die boost converter takes significant die area, which increases cost. When your product is ultra-cheap (think toys, remote controls and such) these tenths of pennies (or fractions of RMB) matter. In this world it's quite reasonable to sacrifice field upgradeability for cost.
Applying higher programming voltage externally makes the programming of the flash memory faster. Internal voltage generators can only supply limited programming voltage or current so it is slower. In some cases, the internal generators do not exist so applying external voltage when programming is necessary.