Similar but not the same. There is an insane variety of nearly identical MCUs, for example. For an otherwise identical part, you can often get it in half a dozen or more packages. (i.e. the chip has different physical form factors in terms of how it attaches to the PCB, pin type and spacing etc... but the actual die inside is the same) Then there are subtle variants for a given MCU (differing types/amounts/speeds of RAM/Flash/interfaces etc.) The matrix of variants quickly gets out of hand. It's probably easily 10-50x (depending on the part) the number of variants Intel/AMD come out with each year. The autos also typically use parts with an extended temperature range.
For the subtle variants, I believe it's how they price discriminate. It's all the same part, but they use fuses to disable various peripherals. Way easier to do that than to fabricate a whole bunch of different chips.
Most likely when it's moderate/major features. Sometimes it can be as simple as a case of packaging (i.e. fewer pins so they can't bring everything out.) I've always assumed that some of the packaging options were driven by large customers who wanted something just a bit different for whatever reason. (PCB space, power consumption, just to be difficult... who knows)
