What is "high voltage programming", and why do some MCUs use it?
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.