ST should have told us about their motivation.
A Flash bank is a section of Flash that contains one singular hardware to generate the erase/programming voltages. Dual bank means each bank has it's own power block.
Reading/fetching from Flash while programming is not possible. With a dual bank option, you can erase/program in one bank, while running code from the other. Extra hardware means extra costs.
This is useful for field updates of occasional parameter changes, both of which are not use cases for many applications.
